Amazing Stories (1985)

S2E8 Go to the Head of the Class

After a teacher enforces an odd punishment, two students decide to take matters into their own hands by using a spell taken from a rock song played in reverse. However, their plan backfires spectacularly when the spell doesn’t go as intended. In their desperation to remedy the situation, they try to cast a different spell, only to inadvertently decapitate their teacher in the process.