Ok i will try to explain it the best I can, but its probably not going to turn out good.

The very opening scene is a dance scene. All I remember about it is there is also a guy with her (I think some classmates as well) and some very intrigueing music followed by the girl laying on her side, naked, with a guy behind her.

A mentally ill girl is jealous of the girl from the opening scene (throughout the movie she becomes more jealous I think) and she kills her and hides her dead body after a struggle between them. Before that happens, the mentally ill girl invites the pretty girl over for something which I can't remember, then she kills her.

That is all i really remember about the movie. Hope someone can pick away at this and help!