I really need help finding this horror film I watched when i was a kid. Can someone tell me the name?

I don't remember everything that happened. But I do remember a few scenes. In the movie these teenagers had went to this place and these little small slimy worm like things came out and went inside there bodies. This turned you into a monster but you could turn into a monster or stay in your human form. Only a few people didn't get them. I remember there was a scene when the main character who was a boy, had everyone over at his house( I think) and he had a substance sand like. If you put it up your nose and it came back out, that meant you were a monster. I remember the boys girlfriend was one of them and i remember a scene were she was crawling on her head using tentacles. The girl chased the boy onto a school bus but he escaped through the emergency exit on the ceiling. After that the girls that were inside of the school were sitting in the gym(I think). After that one of the girls appeared to be a monster and locked the other girl up. The boy came inside and the girl who was the monster walked around naked and she was talking to the boy knowing he was near. The boy was hiding behind lockers from her. Eventually the girl went back into her monster form and chased the boy around the school and into the locker room. Then the boy killed her by stabbing her in the eye with one of those test tubes of the sandy stuff. Can someone please tell me the name of this movie. I really want to watch it again.