Horror movie with two girls, one of whom is a monster who tries to kill the other in a pool

Question

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.

Answer 1

This is probably The Faculty. From the Wikipedia summary:

Casey returns to the gym, where Marybeth reveals herself to be the queen, having faked taking the test earlier. She attacks Stokes, who is stunned at the revelation. Casey and Stokes flee to the swimming pool where Stokes is injured and infected. Zeke and Casey go to the locker room, where Marybeth reverts to her human disguise, naked. She explains that she is taking over Earth because her own planet is dying. Marybeth transforms back to the queen and renders Zeke unconscious by throwing him into a locker. Casey takes the drug and tricks the queen into following him into the retracting bleachers, thus trapping her. Casey then stabs the drug into the queen's eye to kill it.

Answer 2

You may have a look at the movie Jennifer's Body. I remember Megan Fox trying to kill Amanda Seyfried in a pool.