Night of the Creeps (1986) IMDb
An alien experiment is ejected and crashes on Earth, where it eventually causes an infestation of extraterrestrial slugs that breed inside and control the brains of their victims. They enter by the mouth. This all happens in an American good-nerds-vs-evil-frat-boys environment, and the love interest gets to work the flamethrower.
Meta-heavy horror comedy. Class B all the way.