Cruel Intentions (1999)

"In the game of seduction, there is only one rule: Never fall in love."

Plot Summary

Slaking a thirst for dangerous games, Kathryn challenges her stepbrother, Sebastian, to deflower their headmaster's daughter before the summer ends. If he succeeds, the prize is the chance to bed Kathryn. But if he loses, Kathryn will claim his most prized possession.