The Faculty (1998) ending / spoiler
Josh Hartnett discovers that his drug (which is actually made mostly of caffenine pills) kills the alien since they seem to be waterbased. Clea DuVall believes that if you find and kill the head alien, everyone will be okay. Turns out that MaryBeth is the head alien. Casey kills her, Casey and Delilah hook up, so do Clea DuVall and the no longer jock, josh hartnett joins the football team and everything else goes back to normal.
Submitted by MovieGeek