What is the title of this old, kind of cheezy horror movie?

It is about aliens that take over a high school, and some of the kids come up with a substance that kills the aliens. They put the substance in Bic pens, then, to prove they are not aliens, each one has to snort some of the substance. A blond girl fakes snorting the stuff, because she is an alien, but you don't find this out until later. At the end, they are trapped inside the gym, and the blond girl has turned into a huge monster, and the main hero is fighting her from behind the bleachers. It isn't really scary, and some of it is pretty funny, like when they try to kill one of the teachers. I can't remember whose in it or the name of it. Any help would be great. Thanks!