You're a 15-year-old girl buying condoms at a drugstore because you think you might have sex with your boyfriend. At the counter, you realize the cashier is the guy who sits behind you in homeroom. What do you do? Smile bravely? Or bolt for the door?

That's a scene many educators believe needs to be rehearsed frequently in high school AIDS education classes if such programs are to succeed.

Although only 530 youths between 13 and 19 have AIDS, according to the Centers for Disease Control, the number infected with HIV is far greater, possibly 100 times higher. In addition, many people in their twenties who are now being diagnosed with AIDS were probably infected as teenagers, authorities say.

AIDS education has expanded rapidly in the 3 1/2 years since former Surgeon General C. Everett Koop, in his landmark report on AIDS, recommended that sex and AIDS education begin "at the lowest grade possible." Surveys since then have found that teenagers are knowledgeable about how AIDS is transmitted and that school programs have helped raise the level of knowledge. But there is very little information on which programs, if any, actually change teens' behavior.

A report issued last month by the General Accounting Office and several other studies question the effectiveness of school AIDS efforts. Critics contend that while most may provide sound factual information, they do little to influence -- or change -- teenagers' sexual behavior.

And the problem, recent studies have repeatedly demonstrated, is that a surprising number of adolescents engage in behavior that could put them at risk for AIDS. According to preliminary data from CDC's 1989 survey, 58 percent of the nation's high-school students have had intercourse, about 21 percent have had four or more sexual partners and 2.7 percent have used intravenous drugs. Yet according to GAO, only about one quarter of them use condoms.

A Columbia University study has found that high-risk youths -- sexually active runaways and young gay males -- still engage in sex with multiple partners -- without condoms -- despite an extensive knowledge of AIDS. Mary Jane Rotheram-Borus, associate professor of clinical psychology at Columbia, said the youths in her ongoing five-year study resemble typical high-school students in that they are rarely prepared to discuss the need for condoms or for postponing sex in crucial situations. "Boys and girls don't talk about sex. The boy's hands are all over the girl, and it just moves on from there."

Two thirds of the nation's public-school districts provide formal AIDS education, the GAO found. If special assemblies and other one-time activities are counted, 79 percent of school districts offer some form of AIDS education, reports the National School Boards Association. Most programs began after 1987, when CDC first distributed $11 million in AIDS education funds to state education departments and local school districts. While federal spending is up fourfold since then, the GAO concluded that school-based AIDS efforts are "not yet commensurate with the epidemic's potential for disaster."

Part of the problem, some public health officials say, is that health teachers euphemistically refer to the "exchange of bodily fluids," not to blood and semen and sexual practices, like anal intercourse, that are likely to spread the virus. And few provide students with opportunities in which to practice responses by role-playing scenes such as buying condoms or just saying no.

On average, a student will receive a total of six hours of instruction about AIDS and other sexually transmitted diseases between grades 7 and 12, according to a survey published last year by the Alan Guttmacher Institute, a New York research organization that studies birth control and sexually transmitted diseases. "I would question whether anybody reasonably thinks that level of education, in the context of the broader life these kids are living, is going to . . . have much effect," said Jacqueline Darroch Forrest, the institute's vice president for research.

There are other important gaps in public schools' AIDS instruction, surveys show. Very few school districts offer AIDS instruction in the 11th and 12th grades, when students are most likely to be sexually active, according to the GAO. Apparently, that's because AIDS instruction is generally concentrated in a standard health course offered in junior high school.

Even when traditional sex education does a good job of conveying the necessary information about AIDS, critics say, schools must do more. Adolescents, they argue, need to learn social skills for translating knowledge into action, especially as they grapple with their burgeoning sexuality.

When students see a film about the biology of AIDS, "that has no effect on their behavior in the back seat of a car," says Robert Selverstone, a psychologist who teaches a course in human sexuality for 11th and 12th graders at Staples High School in Westport, N.Y. Selverstone is president of the board of directors of the nonprofit Sex Information and Education Council of the U.S. Its survey of public-school curricula last year criticized many school AIDS programs for teaching technical information about HIV transmission "at the expense of a frank, thorough discussion of preventive practices."

Selverstone and other educators say the best way to influence students is to have them play out roles in situations where they might put themselves at risk of infection. In the skit, "Time for a Rubber," pilot-tested in 26 school districts in California, students act out a scene in which a teenage couple has just had sex without contraception. One student tries to persuade the other to use contraception while the hesitant partner spouts scripted lines like, "Let's talk about it next time, okay? Don't spoil the mood."This kind of implicit acceptance of teenage sexuality often generates opposition from parents who favor abstinence. It also makes many teachers uncomfortable. Sex education teachers "are generally nervous about the reactions of people in their communities," said Forrest, author of a nationwide survey in which teachers cited pressure from parents, administrators and community groups as the chief obstacle. That is especially true when it comes to sensitive topics such as contraception and homosexuality.

In the belief that students are more likely to listen to peers than to teachers when it comes to sex, some schools and colleges are training students to teach AIDS prevention to each other. Students at Ballou High School in Southeast Washington deliver their own rap songs about the dangers of AIDS to youths at other District high schools.