You can't say "Private School" doesn't go out of its way to appear impudent. Now soliciting the lowest common denominator of smutty amusement at several area theaters, the movie begins by borrowing Harry Nilsson's song "You're Breaking My Heart" to accompany the credits. Predictably, nothing becomes the picture so much as the shock diminuendo effect of this obscenely pithy but funny lyric--"You're breaking my heart, you're tearing me apart, so f--- you . . ."

It's all downhill from there, as the filmmakers struggle to sustain 90 minutes of slovenly footage on prehistoric sex jokes and the spectacle of the bouncing bosoms that belong to the student body of Cherryvale Academy for sluts and are avidly ogled by the student body of adjacent Freemount Academy for wimps.

To the primitive extent that anything gets under way in the screenplay by Dan Greenburg and Suzanne O'Malley, "Private School" concerns itself with the intention of these young steadies, Phoebe Cates as Chris and Matthew Modine as Jim, to consummate their puppy love at a weekend kennel, the Hotel d'Amour. The only source of complication is Betsy Russell as Chris' troublemaking classmate, Jordan, who contemplates making a play for the mysteriously coveted Jim. This scheme provides a couple of excuses for Russell to strip, drink lewdly from a pop bottle and jiggle about on horseback (gratuitous cultural note: she's a granddaughter of Max Lerner), but it fails to break up the insufferable lovebirds.

In the meantime, Jim's zany pal Bubba, a kind of Jewish juvenile version of Curley the Stooge, impersonated by a ravenous mug named Michael Zorek, kills time by trying to infiltrate the girls' dorm. His most elaborate attempt involves Jim and another friend in a three-sided transvestite act. Curiously, Bubba turns up in the same drag costume at the Cherryvale commencement ceremony, suggesting that inversion may have more than a fleeting charm for this effusive lout. The girls conclude the merriment by lifting their gowns and mooning the faculty, and there you are, if you're daft enough to confuse "Private School" with a movie date. PRIVATE SCHOOL

Directed by Noel Black; written by Dan Greenburg and Suzanne O'Malley; edited by Fred Chulack A.C.E.; director of photography, Walter Lasally; produced by R. Ben Efraim and Don Enright. A Universal release. Rated R. THE CAST Christine .... Phoebe Cates Jordan .... Betsy Russell Jim .... Matthew Modine Bubba .... Michael Zorek