Slaughter High (1986)

Nerd Marty Rantzen is an outcast at Doddsville County High School. He is seduced by his sexy classmate Carol and they go to the girls' locker room to have sex. However, it is a cruel April Fool's prank from eight mates who record him naked and humiliate him until the moment the coach stops them. Martin goes to the laboratory to work on his science project and two mates give him a marijuana cigarette with something mixed in that makes him feel sick. While he goes to the bathroom, Skip messes with his substances. Marty returns to the lab and accidentally provokes a fire in the place and is disfigured by nitric acid. Years later, eight former students receive an invitation to a reunion in the high school on April 1, and Carol calls her friend Susan about it. They find a derelict locked school, but when it starts to rain, they break into the school and learn from former janitor Digby that the school has been abandoned for five years and will be demolished. Further, now he is the caretaker and will leave the place at midnight. They find a room with beer, cakes, and candies and decide to celebrate the reunion. But soon they learn that this is a malignant plot of Marty seeking revenge and they are locked in the school.