Whats this movie called?

It was an alien movie, I think the main character showed his science teacher the alien thing he found, and I guess they decided to put the alien thing in an aquarium. the alien grew, and I think they had tentacles. Anyway, they started to take over people. There was a drug that some students took, and it killed the aliens. A student who was controlled by an alien pretend to take the drug. a student might have killed a teacher with scissors at one point, or visa versa. That's all I remember. Thanks for your help.