Could be Night of the Creeps.
A nerdy college (not highschool) guy has a crush on a frat dude's girlfriend. He tries to join the frat to impress the chick and steals a body from the college's science lab as part of his hazing. Unfortunately, the body has space slugs in it that turn people into zombies. The frat boys all die when their bus wrecks and the space slugs get into them. The nerdy guy and the chick kill most of them with a shotgun and a flamethrower.
There's a lot more to the plot, but that's what corresponds with your memory.
http://www.imdb.com/title/tt0091630/