A sex comedy for slow learners, "Private Lessons," opening today in area theaters, struggles to sustain feature-length playfulness with a one-joke game of seduction. Will the shy, gauche, overprivileged teenage hero, Philly, allow himself to be sexually initiated by the alluring new French housekeeper, Nicole?

There's no plausible reason why Philly shouldn't revel in his good fortune. And no one to disapprove either: His mother is deceased, his father is away on a cheerfully promiscuous business trip, and Philly has the run of the family mansion. The setup is so conducive to hedonistic wish-fulfillment that it's a pity writer Dan Greenburg and director Alan Myerson lacked the wit to capitalize on it.

Philly, played by Eric Brown, should have it made in the shade all summer long. Instead, he persists in running away from the insinuating Nicole, played by Sylvia Kristel, the erstwhile Emmanuelle of soft-core movie fame. This could have been an appealing fantasy about a bright, bashful kid who hits the erotic jackpot. Instead, we get an incorrigible little nincompoop who's too dumb to respond to lavish enticement.

There's supposed to be a snake in the grass who masterminds the seduction -- Howard Hesseman as a mercenary chauffeur who's exploiting Nicole, an illegal immigrant, in a blackmail scheme. However, it's impossible to take this aspect of the plot seriously. A superfluous character, Hesseman is far more of a nuisance than a threat. The spectacle of Philly bolting in nervous embarrassment from Nicole's inviting presence is played as a running gag, with follow-up scenes in which he tells his disbelieving best friend Herman, played by an engaging porky named Patrick Piccininni, how he almost scored.

If you can't buy the idea that Nicole is being manipulated by the villainous chauffeur, her sexual fixation on Philly may seem difficult to rationalize. But she is a long way from home and it is a long drive to town.