Barry Schlafstein is a junior at Cornell University and earns honor grades, but with the competition fierce he still can't be sure he'll get into medical school.

This summer he's paying $350 and spending several days a week in a Connecticut Avenue office building trying to improve his chances. Along with about 1,000 other students, Schlafstein is preparing for an exam--in his case the Medical College Admission Test (MCAT)--at the Stanley H. Kaplan Educational Center.

In Schlafstein's opinion the time and the money are well spent.

"There are so many people taking these test preparation programs now that if you don't go, you think you're at a disadvantage," said Schlafstein, who usually rides a bicycle to Kaplan's center from his home in Potomac. "It's worth having someone pull things together to help you study . . . . It's a shame if you can't afford to pay for this. But really, compared to all the costs of medical school and what comes afterward, it's a drop in the bucket."

Spurred by the belief that tests hold the key to their future and that regular classes haven't taught what they need to do well, an increasing number of students are enrolling in special preparation courses for admissions and licensing exams.

Not only are there commercial courses offered by chains such as Kaplan's and hundreds of small local firms, but many colleges and high schools, which used to look askance at "teaching to the tests," are now giving test prep courses of their own.

The classes are aimed at dozens of exams: Scholastic Aptitude Tests (SATs) for getting into college; admissions exams for law, medical, and graduate business schools; licensing exams for medicine, law, architecture, and accounting. There are even classes for the Secondary School Admission Test (SSAT) for entering private high schools. The programs range from a few hours to several months, from no cost to $1,000.

What they accomplish is a matter of considerable debate. But after actively discouraging it for decades, many academic groups that sponsor the tests now concede coaching can help, though they still say it is unnecessary and warn that its quality varies greatly.

Last winter the National Academy of Sciences concluded that while short-term coaching produces insignificant gains, long-term programs, which resemble remedial or review courses, can have a substantial impact.

"A lot of the problem is that we've all gotten fouled up with the semantics," said Alexandra Wigdor, study director for the National Academy report. "If the tests deal with logical and reasoning skills, these can be taught. It's a matter of how much time we spend and how well we teach them."

For Stanley Kaplan, who started coaching for tests in his Brooklyn basement in 1938, the boom has transformed a small business into a nationwide chain that even his competitors concede is by far the largest in the field. Last year, Kaplan said, his 111 schools had 74,000 students, including about 3,000 in Washington. Receipts topped $22 million. Both enrollment and revenues have more than doubled since 1978, Kaplan said, mostly with students trying to get into professional schools.

Of 43,000 candidates who took the Medical College Admission Test last year, for example, about 40 percent enrolled in his courses, according to Kaplan. So did about 20 percent of the 107,000 taking the Law School Admission Test.

"I say, 'Thank God for the tests,' " Kaplan declares, "not for me so much but so that students will push for something and be motivated to learn.

"In America there's nothing wrong with competition. There's nothing wrong with trying to do the best you can. And that's what I try to help students do."

Not everyone is so pleased with the growth of test preparation courses.

"Some students take these courses, others feel they should, and the whole thing spirals up until it becomes the norm," said Robert J. Munn, premedical advisor at the University of Maryland. "In a competitive world people are always looking for the little additional edge, and they'll pay for it. Pretty soon a whole industry builds up feeding on those fears and aspirations. It can make you shudder."

In 1979 coaching schools received some significant help: a well-publicized report by the staff of the Federal Trade Commission that said Kaplan's classes for the SAT had produced gains averaging 25 points each on both the verbal and math sections of the test (on a scale of 200 to 800 points). The agency's Boston office said the gains were twice that large, the same as Kaplan reports, but the final FTC study said this data was "seriously flawed" because it did not take into account the background and self-selection of Kaplan's students.

About a year later, the SAT's sponsor, the College Entrance Examination Board, which had warned students against coaching for two decades, issued a more neutral statement. It said long-term "special preparation" might raise scores as much as 25 to 30 points on each section. The Law School Admission Council, sponsor of the LSAT, also softened its anticoaching stand.

For Kaplan, 63, the FTC report and the test makers' changes in policy brought vindication after decades of suspicion and disdain. He was even invited to speak at a College Board meeting, and received a respectful reception.

"I consider myself an educator who is providing something which many students need because they're not getting it in the schools," said Kaplan, who often sounds like an enthusiastic old-line teacher with a penchant for definitions and practical advice. "That's what interests me, not the dollars."

Kaplan, whose New York headquarters is around the corner from the College Board office, said he agrees with the board and the Educational Testing Service, which develops the exams, that cram courses of a few hours or a weekend can't do much to raise scores.

"That's not my course," he said. "I give students the equivalent of six months work, not just test-taking techniques . . . . We don't make any guarantees. But for those who have some potential and do the work, we can improve their skills, their reasoning skills. When students have stronger skills, they get higher scores on the tests.There's no trick involved."

To critics of the tests, led by the National Education Association, the country's largest teachers' union, evidence of coaching success throws doubt on the value of the SAT as a stable predictor of college performance.

"If performance goes up with coaching, then you don't know how long the effect will last," said Carol Norman, an NEA research specialist. "You're not measuring any 'scholastic aptitude,' just how well something has been taught."

College Board officials rejoin that that the test measures the "developed abilities"--verbal reasoning, math problem-solving, vocabulary and reading comprehension--needed for academic work.

"If Kaplan develops those skills, the scores will go up," said Lawrence E. Gladieux, executive director of the College Board's Washington office. "The tough question is whether a student can get a significant return from these courses . . . . We can't say there's nothing to them . . . . On the other hand, if a student had the motivation, worked hard in school, used our preparation booklets, and didn't spend all that money on Kaplan's classes , wouldn't he get the same results as with Stanley Kaplan? I don't know, and he can't say either."

For $325--plus a $50 deposit for home study materials--a student in Kaplan's SAT prep program gets 11 class lessons of four or five hours each, homework assignments of about an hour a night for 10 weeks, and practice exams with about 200 hours of tape-recorded explanations to work on weaknesses.

In Kaplan's Washington school, at Connecticut Avenue and Van Ness Street NW, the tapes are available in a listening room 71 hours a week.

The courses for MCAT and most other graduate school entrance exams have eight class lessons instead of 11 but otherwise follow the same format. For the accountants' and physicians' licensing exams there are hundreds of hours of tapes, sample tests and review books, but no live classes. Kaplan says students prefer it that way.

In all the programs that do have classes, most sessions start with a short lecture, then a half-hour test modeled in part on the actual exam. The rest of the time is spent explaining the answers, wrong ones as well as right, stressing the reasoning involved and logical short-cuts.

Most of the instructors are young--some in law or medical school themselves. All have scored very high on the tests they teach.

Last year about 7,000 students across the country took Kaplan's SAT prep classes, up from 5,000 the year before. This was still just 1/2 of 1 percent of the 1.4 million taking the SAT nationwide. Many thousands more took other commercial programs or SAT prep courses in their schools.

In Montgomery County, for example, about 1,900 students enrolled in after-school SAT prep classes, costing $35 for 10 two-hour sessions over five weeks. Fairfax County schools, which started similar classes for the first time last fall, had 1,500 enrolled.

In addition, several Fairfax high schools offered a one-semester "verbal skills" course as an elective during the regular school day that prepared students for the verbal part of the SAT.

"Special preparation courses for the SAT are cropping up in schools all over the country," said Scott D. Thomson, executive director of the National Association of Secondary School Principals, which is preparing a model course of its own. "Our position is that students need the skills on these tests, and the schools should make sure they learn them. If parents want to go out and buy a special course, that's fine. But we think it is the schools' responsibility to offer it no matter what anyone can afford to pay."

Kaplan, who went to City College of New York when it was free and highly selective, acknowledges that his courses give an advantage to those who can afford them.

He said he gives reduced-fee scholarships to about 10 percent of his students. Of the others, he said:

"Why shouldn't parents do the best they can for their children? You know, some people can afford to live in wealthy suburban neighborhoods and send their children to very good public schools. Others pay $4,000 a year for the advantages of private schools. What we offer costs $325. Sometimes I call us a poor man's private school."

Although some critics suggest that Kaplan-style prep courses should be offerred to students with poor grades and very low test scores, Kaplan says it is unlikely they would do much good.

"We've tried some classes with very disadvantaged students," he said. "But they were so much behind, we couldn't get very far. Look, if a student doesn't know how to add or multiply, how can we teach him algebra? If he doesn't know the meaning of stubborn, how can we teach adamant? Our job is to review and synthesize and put things together. We can't start from scratch."