How could they do this to Jennifer Jason Leigh? How could they put such a fresh and cheerful person into such a scuz-pit of a movie? Don't they know they have a star on their hands? I didn't even know who Leigh was when I walked into "Fast Times at Ridgemont High," and yet I was completely won over by her. She contained so much life and light that she was a joy to behold. And then she and everybody else in this so-called comedy is invited to plunge into offensive vulgarity.

Let me make myself clear. I am not against vulgarity as a subject for a movie comedy. Sometimes I treasure it, when it's used with inspiration, as in "The Producers" or "National Lampoon's Animal House." But vulgarity is a very tricky thing to handle in a comedy; tone is everything, and the makers of "Fast Times at Ridgemont High" have an absolute gift for taking potentially funny situations and turning them into general embarrassment. They're tone-deaf.

The movie's another one of those adolescent sex romps, such as "Porky's" and "Animal House," in which part of the humor comes from raunchy situations and dialogue. This movie is so raunchy, however, that the audience can't quite believe it. I went to a sneak preview thrown by a rock radio station, and the audience had come for a good time. But during a scene involving some extremely frank talk about certain popular methods of sexual behavior, even the rock fans were grossed out. There's a difference between raunchiness and gynecological detail.

The movie's cast struggles valiantly through all this dreck. Rarely have I seen so many attractive young performers invited to appear in so many unattractive scenes. Leigh, for example, plays a virginal young student at Ridgemont High. She's curious about sex, so the script immediately turns her into a promiscuous sex machine who will go to bed with anybody. And then her sexual experiences all turn out to have an unnecessary element of realism, so that we have to see her humiliated, disappointed, and embarrassed. Whatever happened to upbeat sex? Whatever happened to love and lust and romance, and scenes where good-looking kids had a little joy and excitement in life, instead of all this grungy downbeat humiliation? Why does someone as pretty as Leigh have to have her nudity exploited in shots where the only point is to show her ill-at-ease?

If this movie had been directed by a man, I'd call it sexist. It was directed by a woman, Amy Heckerling -- and it's sexist all the same. It clunks to a halt now and then for some heartfelt, badly handled material about pregnancy and abortion. I suppose that's Heckerling paying dues to some misconception of the women's movement. But for the most part this movie just exploits its performers by trying to walk a tightrope between comedy and sexploitation.

In addition to Leigh's work, however, there are some other good performances. Sean Penn is perfect as the pot-smoking space cadet who has been stoned since the third grade. Phoebe Cates is breathtaking as the more experienced girl who gives Leigh those distasteful lessons in love. Judge Reinhold has fun as a perennial fast-food cook who rebels against the silly uniforms he's supposed to wear. Ray Walston is suitably hateful as the dictatorial history teacher, Mr. Hand. But this movie could have been a lot more fun if it hadn't chosen to confuse embarrassment with humor. The unnecessary detail about sexual functions isn't funny, it's distasteful.

Leigh looks so young, fresh, cheerful, and innocent that we don't laugh when she gets into unhappy scenes with men -- we wince. The whole movie is a failure of taste, tone, and nerve -- the waste of a good cast on erratic, offensive material that hasn't been thought through, or maybe even thought about.