Cruel Intentions (1999)
Synopsis: "Slaking a thirst for dangerous games, Kathryn (Sarah Michelle Gellar) challenges her stepbrother, Sebastian (Ryan Phillippe), to deflower their headmaster's daughter (Reese Witherspoon) before the summer ends. The prize, if he succeeds, is the chance to bed Kathryn. But if he loses, Kathryn will claim his most prized possession. Sebastian takes up the dare but soon learns there's more to the bargain than meets the eye."
Synopsis compliments of Netflix