The Willies(1990)

While trying to scare each other by telling bedtime horror stories in a tent, Michael, Kyle and Josh recount a variety of tales of a fat KFC-eating woman eating a fried rat, an old man dying in a haunted house, an old lady putting her poodle in a microwave, a nerd feeding his teacher and school bullies to a monster which turns out to be the school janitor, and a fat child's obsession with dead flies, which soon gets serious when his flies become powered by manure.