Cruel Intentions (1999)

A tale of betrayal centering on two wealthy, manipulative teenage stepsiblings from Manhattan's uppercrust. It's summer break, and Kathryn has been dumped by her beau, Court Reynolds, for the innocent Cecile. Desperate to get even, Kathryn challenges Sebastian to ruin Cecile by deflowering her--thus humiliating Court by delivering Cecile to him as damaged goods. Sebastian has pretty much 'had' all of the girls in New York City up to this point, and he's gotten a bit bored of it all. Though this is too easy a conquest for him, he obliges. He sets his sights on a higher challenge--the headmaster's daughter, Annette, who recently wrote an article in 'Seventeen' Magazine about how she intends to stay pure until she marries her boyfriend. Sebastian bets Kathryn that he can seduce the chaste and pristine Annette. Kathryn believes it impossible, and agrees to the wager--and if Sebastian succeeds, Kathryn must give him a night of unbridled pleasure, something he's wanted since their parents got married.