IF YOU DON'T like this movie, no one will Class may have something for everyone, but for the Harvard set it's pure ego-massage. Sure, the plot weaves through prep school, sexual initiation and the birth of a friendship but it marches slowly and inescapably to one place. Harvard And when a movie tells you all prep school ambition leads to where you sit, what could be more fun than a wistful look at how you got there or might have.

In Class, two trains to Harvard Square meet in the way station of Vernon Academy, an Illinois boys' prep school. The express arrives in the form of Square "Skip" Ellsworth Burroughs IV (Rob Lowe), an academy lifer who, well, the name says it all. But by senior year, the local chugs in and Skip finds himself rooming with Jonathan Ogner (Andrew McCarthy), a bright it somewhat unworldly scholarship student First day out, the preppie picks on the nerd and Class seems to be shaping up into a celluloid bildungstoman

But a funny thing happens on the way to the saccharin vat, and Class emerges more intriguing, if no less sweet, than the standard Hollywood fate What sets this movie apart, strangely enough, is plausibility Strangely, because at first glance credibility is the last thing you'd expect from a movie that boasts an across-the-tracks camaraderie, ubiquitous prep school antics, and an inadvertent love affair between Jonathan and his roommate Skip's mother, played by Jacqueline Bisset.

It's a plot with "formula" written all over it, yet director Lewis Carlino successfully works Class into an enjoyable and believable two hours. Much of the credit goes to the actors, who, despite some painfully stagy lapses, sustain comic realism throughout. Except for Bisset (whose education of Jonathan is so captivating that you never wonder why she's lavishing it on a 19-year-old) the adult characters are largely unconvincing stereotypes, but the academy inmates are almost all outstanding. Lowe, fresh off the set of The Outsiders, and film-newcomer McCarthy work well as Vernon's dynamic duo, constructing a close roommate-friendship that is first destroyed, then rebuilt in the wake of Jonathan's affair. And John Cusak steals more than one scene as Roscoe, the school's butter-wouldn't-melt-in-your-month darling who feeds inside information to "the gang."

From the opening bus station farewell between Andrew and his parents, instantly recognizable to anyone who's ever taken a trip any where, to the Halloween party alligator who greets people, "Hi, want to buy a shirt?" Class is an eclectic collage of moments. Jonathan's romance includes an outrageous glass elevator seduction. "I love elevators. I love the way they go up and down." Some how school existence and his nature sexual involvement forces reassuringly to the son, not the mother.

And similarly throughout this "romantic comedy," the humor is very much divorced from the romance--a division that works to enhance both elements of the film. The tension between his boyish high school existence and his mature sexual involvement focuses Jonathan to reconcile his two halves; eventually he becomes both more adult among friends and more lighthearted with Skip's mother, Ellen. But because Carlino does not indulge in psychology sleight of hand, making Jonathan's emotional maturation too rapid-fire, he never presents the relationship between the boy and his best friend's mother as more than it could credibly be. Once Jonathan discovers Ellen is in fact Ellen Burroughs, his loyalties fall resoundingly and reassuringly to the son, not the mother

All of which keeps Class exactly where it should be, in a friendly unpretentious spot somewhere between adolescence and manhood. It's a gangly film, awkward in spots but lovable overall. Perhaps its greatest flaw is that it tries too hard to be loved--and while it does escape looking canned there are a few too many tweed jackets and Shetland sweaters. One might wonder if there ever was, or will be, a prep school quite so preppie as Vernon Academy.

Actually, there is. But the several scenes of Class filmed in Harvard Yard last fall never made it into the finished product.