"Class," a sexual disillusion acted out at the prep school level, would be represented far more accurately by the one-word title "Crass."

Opening today at area theaters, this latest superfluous Spawn of Animal House contents itself for the most part with dormitory prankishness. It's not until a farfetched sexual liaison is introduced to complicate the incessant goofing around that the blend develops insufferably hypocritical and nauseating tendencies.

When Jonathan (Andrew McCarthy), a senior newcomer, arrives at ritzy Vernon Academy in suburban Chicago, he's instantly made the butt of a humiliating practical joke by roomie Skip (Rob Lowe), a fun-loving rich kid. Although shy, sensitive Jonathan would appear to be a sleepwalking invitation to practical jokers, he promptly turns the tables on jolly, extroverted, cutie-pie Skip, cementing a fast friendship. Further hilarious adventures, with Jonathan typically making a fool of himself, enhance the camaraderie. Recruited by Skip for a double-date with girls from neighboring Foxfield, Jonathan ends the evening prematurely by upchucking in the back seat; at a dance committee meeting at Foxfield, he accidentally starts a falling dominoes sequence of clumsiness that concludes with the committee room a mess and one coed flashing her breasts.

The second mishap supposedly bars Jonathan from the dance (a costume affair, which irrepressible Skip attends disguised as Jesus, bearing a cross). At the urging of his roomie, he searches for consolation at a Rush Street saloon called the Free and Easy Club. After enduring a couple more humiliations attempting to ingratiate himself with likely-looking loose women, Jonathan is picked up by an extraordinarily glamorous specimen, in the person of Jacqueline Bisset, who proceeds to ravish him in the privacy of a glass elevator while making playful innuendo about the relative merits of "going up" and "going down."

Jonathan's remarkable stroke of luck is sustained even more implausibly for several thrill-packed weekends. For the sake of convenience, the gorgeous cradle-robber seems to buy his fib that he's a Northwestern student, but she suddenly bolts during a weekend jaunt to Manhattan after sneaking a belated peak at his wallet and gazing upon a Vernon Academy ID. The disappearance of his beautiful benefactor leaves Jonathan puzzled and frustrated for weeks, but the mystery is cleared up with a shock when he accepts an invitation to spend Christmas vacation at Skip's and meets his roomie's dad and mom for the first time.

Unable to imagine sustaining sources of dramatic conflict in the prep school setting itself, the writers spring their non-surprise about Bisset's identity in order to generate some artificial suspense to accompany the prevailing sniggers. The idea that a woman this posh would be preying on boys as gauche as Jonathan strains credulity from the outset, and the incredible coincidence angle strains it beyond the breaking point.

With no apparent help from the writers, Bisset tries to make some subliminal sense of her character's attraction to Jonathan by projecting a slightly manic personality, a mood of gaiety verging on desperation, during their romantic encounters. There's never a scene in which her character is granted the luxury of explaining herself to lover, son, spouse, shrink, mirror or what-have-you, and the filmmakers don't seem to realize that it's hypocritical to get solemn about her lustfulness when they've been setting a tone of slaphappy hedonism all along. During the Christmas interlude, she virtually orders Jonathan out of the house, then inexplicably plagues him with lovelorn phone calls to the dorm when school resumes. "You don't know anything about me," she pleads during one of these strange transmissions, calling attention to the fact that the writers haven't bothered to tell us anything either.

A few moviegoers probably recall that Bisset was playing a much more appealing version of her "Class" role 15 years ago in a slight but good-natured sentimental comedy called "The First Time," in which she graciously elected to initiate a virginal teen-ager played by Wes Stern. Now we find her providing the same service in a coarsened context. This is the sort of career progression calculated to document the depressing fact that most careers tend to get nowhere in particular. CLASS

Directed by Lewis John Carlino; written by Jim Kouf and David Greenwalt; director of photography, Ric Waite; music by Elmer Bernstein; executive producer, Cathleen Summers; produced by Martin Ransohoff. An Orion Pictures release. Rated R. THE CAST Ellen . . . . Jacqueline Bisset Skip . . . . Rob Lowe Jonathan . . . . Andrew McCarthy Mr. Burroughs . . . . Cliff Robertson Balaban . . . . Stuart Margolin