Happy Days S09 E07 Fonzie, the Substitute

"Those were the days of the 1950's...filled with innocence and the promise of even better days to come."

Fonzie substitutes for Roger in his hygiene class, and, as a prank, the students hide the lesson plan and tell Fonzie they're learning about sex. Roger is later informed that he will be fired as a teacher as a result of Fonzie's teachings.