At the southern tip of the District of Columbia, on a small rise overlooking the A Potomac River and Woodrow Wilson Bridge, there are 24 government-owned buildings surrounded by a chain-link fence. Behind that fence live about 500 young men and women. They are watched day and night, awakened for work at 6 a.m., ordered to remove their hats while eating and allowed to leave the center only briefly.

But they are not in jail. At this place, bad behavior is a reason for dismissal.

The youths attend class for six hours a day, training for jobs as airline reservation and railroad clerks, bricklayers, carpenters, cooks and hospital aides. They have a student government. At the end of their classes, they return to air-conditioned dormitory rooms that have desks, perhaps a framed print hanging on the wall.

There is a swimming pool reserved for the sole use of the students, a basketball court and a baseball diamond. There are trips to the Kennedy Center. After the sun sets over the Potomac, there are classes in ceramics and macrame, there are pool tables and there are dances. A new movie is shown each night.

This is not a college or a summer camp. These students, many of them high school dropouts, are paid to attend and study: $40-$100 a month plus a graduation bonus.

This is the Potomac Job Corps Center, one of more than 100 in the United States, one of two in the Washington area and the only one in the District of Columbia. It was established in 1978 in a group of buildings that used to house homeless children assigned to the former Junior Village.

After a stay of six to nine months, the students are expected to leave and use their newly acquired skills to get and keep steady jobs.

Set up as part of Lyndon Johnson's "Great Society," the Job Corps at the end of the 1960s was what one observer called "probably the most unpopular antipoverty program with Congress."

Earlier this year, however, Sen. Orrin G. Hatch (R-Utah), a prominent conservative, called the Job Corps "a government training program that provides jobs and saves more dollars than it expends."

President Reagan's budget cutters apparently agreed with Hatch: The Job Corps program is slated to receive an increase of $67 million in fiscal 1982 over the $560 million it was given in fiscal 1981.

The Services Division of RCA Corp. gets about $5 million of that each year to run the Potomac Job Corps Center under contract with the Department of Labor. RCA takes youths from poor families and tries to turn their lives around. The cost per person per year is $9,440.

Almost all of Potomac's 525 Job Corps enrollees are black; most of them are from the Washington area; two-thirds are male.

Almost all the youths who enroll finish the program. With the help of the Job Corps, more than 70 percent of those who finished during fiscal 1980 found jobs.

Potomac Center Director Nellie Williams, who has spent more than a decade as a Job Corps teacher and administrator, said the key to success is that the Job Corps is a residential program, permitting corps members to become immersed in the habits of the workaday world.

Other reasons for success can be found on a short walk through the center.

Rosella Scott, a burly man who teaches bricklaying and cement masonry, acts like a foreman and dresses his "crew" in the overalls of union bricklayers. Scott has placed 16 Job Corps graduates in the cement masons' union in the last two years.

The same philosophy is at work across the street, where teachers from the Brotherhood of Railway & Airline Clerks (BRAC) train their students to take airline reservations and shunt railroad cars.

At the building's entrance, BRAC's Norman Cyr has placed two time clocks and a rack for time cards. Arriving late earns the card-holder a red circle. Just to the left of the clocks hang two copies of railroad rules and regulations.

"To get them into good habits," explained Cyr, who spent 27 years with the Boston & Maine Railroad before becoming a Job Corps teacher.

Facing the time clocks from the other side of the hall is a display case containing photographs of graduates now working as railroad clerks. One of the most recent is Petey Mills of Forest Heights, Md., who lived in the center's buildings when they were part of Junior Village. On Aug. 10, Mills was hired by the Baltimore and Ohio Railroad at a salary of $15,000 a year.

Other corps members learn to be hospital and child care aides, air conditioner repairmen and business clerks. They all follow the "flip-flop" system, alternating weeks of occupational training with weeks in the classroom devoted to reading, writing and arithmetic. Courses are given on personal hygiene, personal finance, family planning and cooking on a budget.

James White, 19, of Richmond, Va., said he feels the Job Corps is misunderstood by the public. "Many people feel the Job Corps is for dropouts, for people on the streets. It's wrong. The Job Corps is a training program for American citizens."

At the end of each day, corps members return to their dormitories, which are cleaner and better looking than many college dormitories. That was not always so, said David Shinton, the center's business manager.

Proud of what he calls his "humanization" program, Shinton shows his visitor the new color-coordinated lounges, the shower tiles installed by corps members, the new soft lighting on the ceilings.

Nellie Williams has a succinct reason for giving corps members comfortable quarters: "You don't take people out of dumps and put them into dumps."

The Job Corps is open to men and women 16 to 21 years old who are out of school and unskilled. They must be from a "disadvantaged" background; for a family of four, an income of less than $8,450 annually is considered disadvantaged. There is no educational requirement.

There are more applications than places, meaning an average waiting period of four to six months between application and enrollment. More than 1,000 District residents have joined the Job Corps since the end of 1978, when the two Washington-area centers (Potomac in the District and Woodland in Laurel, Md.) were opened.

Corps members who get involved in fights or refuse to obey orders are placed on work details and denied weekend passes. (About 75 percent of the center's corps members go home each weekend.) Repeat offenders are expelled; last year, 68 corps members were expelled.

Sitting on the fence and smoking after dinner, six corps members seem happy with the rules. What bothers them is the cafeteria. "The food be half raw," said one.

Another added: "The cooks come in dirty. If you need an escort, the flies'll take you in."

A third added: "We got a lot of 'Bamas down here, don't like to take showers. They come in from playing ball all sweaty, flop down on the mattress."

Asked what they think of the Job Corps program overall, the group yells together: "Good."

What would they be doing if they had not joined Job Corps? The answers come slowly, more quietly and one by one.

"I'd be locked up."

"I'd be in jail too."

"I don't know. Looking for a job."

"Hustling. Jail."