A COUPLE OF YEARS ago I wrote an article about an 18-year-old who lived on the Lower East Side of Manhattan, one of the most ravaged and evil neighborhoods in the country.

Tom Williams (names of teen-agers have been changed) had spent much of his adolescence on the streets. He had been expelled from a public high school and a private Catholic school. He had been arrested for breaking into a grocery store. He had used a lot of drugs.

But I thought Tom had a decent chance to begin a new life. He had earned an equivalency diploma at an alternative high school, and he seemed eager to take a job, any job that would get him off the streets and out of the neighborhood.

After the article appeared, I got a call from the head of a garment center firm. "I'm going to hire Tom," he said. "I'm going to teach him everything about the business from the bottom up. I'm going to give him a real shot."

Three weeks later Tom's new boss called me again. "We got problems," he said. "Friday night Tom emptied the petty cash box. A half a dozen people saw him do it. I've got to fire him."

As for Tom, he couldn't quite understand why he had lost his job. "I just needed some change for the weekend," he said. "I was gonna return it when I got paid."

Last year I met many young men and women who, like Tom, were trying to navigate the difficult passage from a sad and troubled childhood to a full-time job. And most, like Tom, were sinking fast. Their failure not only demonstrated their own personal shortcomings, but the severe limitations of our national effort to prepare disadvantaged youngsters for a lifetime of legitimate work.

In a national study for a group of foundations, I interviewed more than 150 youngsters in 17 cities. The purpose of the study was to find out which federally and locally funded employment and training efforts had been successful and should be sustained during a period when funding for social programs was being reduced.

There wasn't much mystery about it. The successful training programs were directed toward kids who were in the best shape when they enrolled, kids who were still in school, came from fairly stable families and were not in trouble with the law. With some personal attention, tutoring and vocational guidance, the odds were pretty good that they would finish high school, find a job and enter the mainstream of American society. Among all disadvantaged kids, they would be voted the most likely to succeed.

But what about the others? What about the more than a million adolescents in the United States who are without homes? What about the 1.2 million runaways with an average age of 15? What about the half-million throwaways, youngsters who have been rejected or discarded by their parents and are living on their own? What about the million kids who drop out of school every year?

The short answer is that in most employment and training programs these kids are invisible. The message from Washington to local program administrators is: Move as many kids as you can through the training process as fast as possible. The unwritten message is: Forget the ones with the most serious problems and who need the most help. We don't have the money or the time for them.

"The programs that concentrate on the 'winners,' the kids who have the most going for them, are the ones that get funded," says Milton Zatinsky, director of Coalition for Progress, a consortium of community youth groups in Miami. "It's crazy to think we can move a kid who's been hanging out on the streets of Liberty City for years into a job with a downtown bank in a few months. So we pretend those kids aren't there. We act as if they don't exist."

But they do exist. During the day, they can be found riding the subways of New York, sitting on the park benches of the Boston Common or drinking wine at noon in the parking lot behind a liquor store in Fort Lauderdale. At night, they can be found in the abandoned buildings in the Hough section of Cleveland, in the 24-hour arcades of the Loop in Chicago or in temporary youth shelters in the Mission District of San Francisco.

All of the days of their adolescence take on a certain bleak sameness. As Tom Williams told me, "The first thing, you'd wake up in the morning and go outside in the park. You drink, smile a little bit. That's the main recreation, just gettin' high, gettin' drunk. I used to run drugs, numbers. If you have a bag of herb, you make a few jays, make a few dollars. If you steal a few hubcaps, you make a few dollars -- and you blow it right away.

"The people hangin' out thought only one way. They thought about buyin' new flares next week. Get an ounce of herb, go to a party Friday night. Yeah, some are hanging out, some are still on the corner, some are in jail. If you go to jail, that's another medal. Another medal of acceptance to the crowd."

Eventually, those with the keenest sense of self-preservation reach out for a lifeline. For many of them the nearest and most promising lifeline is the local training and employment program.

When a kid enrolls in these programs, he has to answer a few questions about his family income, his grade levels in math and English and the number of years of school he completed. There are no questions about what he did and what happened to him after he left school and home.

Pat Larkins, the head of an association of black contractors in Fort Lauderdale and a 20-year veteran of manpower programs, says, "Most of our programs treat these kids as if they were a group of MBAs in an IBM training course. They don't know who these kids are, and they don't want to know. And the ones they know least about are the ones who've been hanging out on the corner."

Most programs offer a similar mixture of services: basic education to improve reading, writing and computation, the development of certain trade and vocational skills and some assistance in finding a job. Many of the street kids I met have participated in three or four of these programs. They have improved their reading and writing skills and they have learned something about operating a word processor or a cash register or a drill press. But very few last on a job for more than a couple of weeks. They end up as Tom Williams did -- a failure at 18.

Before this generation of dispossessed and estranged youth is sentenced to a lifetime of crime and dependency -- at a cost of roughly $3 billion a year -- there are a couple of things that might be tried.

First, the present pre-employment training system could be changed so that it recognizes who these street kids are, what they've experienced and where they're headed.

Second, these youngsters could be given a fighting chance to hold onto their jobs.

One way to do that would be to establish a national apprenticeship program for our most disconnected youngsters.

Kids come to training programs with heavy baggage: all the anger, fear, defensiveness and suspicion that are the emotional staples of street life. Too often they leave as they entered: hostile, wary and confused.

The most important thing a program can do is to assess the damage a kid has suffered growing up on his own and provide the individual psychological and social support that begins to repair it.

Mark Robinson is a damaged kid, perhaps more damaged and more dangerous than Tom Williams is. He is a tough, streetwise New York teen-ager who sometimes erupts in violence. Mark has passed through several training programs since he left school at 15. He has held a half dozen jobs. His work experience always follows the same pattern: After a few weeks on the job, he gets bored and quits.

Mark's great enthusiasm is writing graffiti. One night I went with him to a trainyard in upper Manhattan and watched him spray- paint a subway car. Afterward, I asked him why he hadn't tried to get a job involved with artwork or as a sign painter. He laughed. "I wouldn't take a job like that 'cause it's no fun. It's just work. There's no danger, no fun in it. Make it a job and you'd ruin it. I have more fun hanging out than any job I could do."

For the middle-class kid growing up gently, fun is not hard to find. He can have fun on a Christmas Eve at home, or at a college dance or on a summer day at the beach. Mark Robinson, long gone from home and school, finds his fun dodging cops in the trainyard or leaping over the third rail or "taking somebody off on the street."

An innovative teacher or counselor might have demonstrated to Mark that some work can be fun -- or at least pleasurable. To do that he would have to know something about Mark's days on the streets and his midnight raids on the trainyards. He would have to know more about what was missing from his years as a street kid -- harmless teen-age frivolity, for example -- and what he wants from a job.

In the three training programs Mark has completed, these critical connections were never considered. All he remembers of training is sitting in a back row, squirming in his seat, counting off the minutes until he could get back on the street. "Just like school," he whispers.

Mark will probably continue to spin, as his friends have, in the whirl of danger and drugs and potential violence. But there's a point where his seemingly inevitable slide toward criminality can be stopped. And that point is when he gets a job.

Many youth employment experts believe that the make-or-break period for kids whose lives have been scarred by parental abuse, drug use and failure in school is the first three or four weeks on the job. Michael Marker, a personnel specialist with Procter and Gamble, says, "If the kid screws up in the first month, if he shows uncertainty or unfamiliarity with the work, if he's distracted by personal problems, the stereotype of failure is reinforced. Everybody draws back from him. His supervisor and coworkers are thinking, 'What are we gonna do to get rid of this burden?' Very quickly you find the kid isolated on the job. His access is shut off."

If a primary objective of our employment and training policy is to provide disadvantaged kids with access to mainstream employment, then the administration and Congress should consider the advantages of establishing a national apprenticeship program.

One way to do that is to assign counselor- mentors to actual worksites where youngsters are learning their jobs. In the early 1960s, Robert Schrank, who directed a youth agency in New York called Mobilization for Youth, assigned counselors to supervise youngsters during their first weeks at Bloomingdale's department store. These kids seemed to do better and last longer on the job than kids who didn't have a mentor, Schrank says.

Currently, in Boston, a nonprofit organization called Transitional Employment Enterprises provides supervisors who act as mentors to women who have been welfare recipients and to retarded persons. More than 85 percent of these new workers, now employed at some 35 companies, retain their jobs after a year. Unions in Colorado, Massachusetts, California and other states also have tested the mentor approach with hard-to-employ youths.

The last thing an employer wants is to act as surrogate parent, social worker and psychologist for a young worker. That's why so many employers are reluctant to hire street kids whom they consider unreliable and erratic. But would employers be as reluctant if a mentor were present who could do all the things a boss cannot or will not do?

Some mentors I've met create a model of a responsible employe by working alongside kids during their break-in period. They intervene when conflicts develop between the youngster and the company supervisor. They arrange for a reference when a kid is looking for another job and they counsel him about possibilities for advancement within the company. Perhaps most important, they make sure kids get supportive social services when personal difficulties, such as family disputes, child care and drug use, divert them from their work.

A few years ago the federal Office of Youth Programs funded a national demonstration program in which mentors or counselors were assigned to the worksites where youngsters were employed. Eight of 10 employers in three cities where the program operated said that the presence of a mentor was the main reason they hired their young workers after a six-month trial period.

Greg Garcia, the owner of a metal door manufacturing shop in Denver, said, "One kid had got arrested in a gang fight. Another was a 19-year-old girl who couldn't figure out what to do with her baby when she was working. As the boss, I don't have the time to get involved in all that crap. The counselor stepped in and straightened out these problems. When the kid screwed up on the job, I'd talk to the counselor about it and she'd go back to the kid. If the kid had some problems with what I was doing the kid could talk to her. She was the bridge."

In Bedford, Mass., Judith Sadowski, a counselor-mentor who has a master's degree in social work, spends a lot of her time washing dishes in a hotel kitchen where young retarded men and women work.

"David, who was 17, was hired as a pot washer," she recalls. "The first week he was here, I was doing 50 percent of his work. The next week it was 35 percent. Gradually, I phased out and let him do the whole job. At the same time I was telling him, 'You're doing a fine job, you just need to work a little faster.' "

She has found that while job performance is important, so too is the youngster's manner and appearance. "You focus on whatever the trouble is," she says. "It might be the way they dress, whether they've shaved, whether their hair is clean and whether they brushed their teeth in the morning. You even have to remind them to use deoderant. That sounds pretty basic, but you know some of our kids haven't had anyone ever tell them that."

Thirty years ago, an Italian or Irish kid who dropped out of school could often find work that allowed him to develop a skill and advance himself. Along the way, he had the support of an informal ethnic network of kin and friends. They eased him into the job, taught him a skill, explained the rules of work, making sure he understood which could be broken and which couldn't, and shielded him against the eccentric and irascible crew boss. His mentors bought him enough time to prove himself.

That's just what this generation of lost kids needs: enough time to straighten out their lives. We can buy that time for them by creating a training system that is responsive to their experiences and needs and that provides mentors who guide and support them on the job.

An apprenticeship system can be costly: it's estimated that $1 billion would pay for mentors for 1 million youngsters. In the long run, though, it may be much more expensive to abandon them to the chaos of the streets.