Heathers (Michael Lehmann, 1988)
If you're after a little more, I loved about the first half hour, but I think it loses something as it goes on. The very end is super. Mean Girls obviously owes a debt, but may actually be the better film (or may not - in any case, it's close). Winona is great, Slater is creepy. Some great themes and ideas going on. Still holds up really well 20 years on, considering how dated it could be.