The Strongest Man in the World (1975)

A school laboratory accident mixes one student's vitamin cereal mix with Dexter Riley's chemical experiment. When the kids decide to dispose of the mess to their neighbor's cow, they learn that the cereal gave the cow the super-strength to give a massively vast supply of milk. When they try it out on themselves, they discover that the stuff gives any human superhuman strength for a few minutes. The school sees this as the thing needed to save their school from closure, as the Dean makes a deal with his relative who owns the company that makes the cereal for financial support, unaware that it was Dexter's chemical which was solely responsible for the strength. When her competitor learn of this deal, he hires two criminals to stop it.