I don't remember much of it because I was so young when I watched it but it’s a horror movie.
I'm pretty sure it's based in a high school. The scene I remember looks like it was in a high school gym swimming pool, and there are two girls. One is some sort of monster and tries to kill her in the pool (the monster girl can transform) then the other girl gets out of the pool somehow and eventually makes her way behind the grand stand and the monster girl follows, turns into some monster thing and the other girl crushes the monster with the grand stand?
That's all I can remember.