Slaughter High

Slaughter High is a 1986 slasher film written and directed by George Dugdale, Mark Ezra and Peter Litten, and starring Caroline Munro, Simon Scuddamore, Carmine Iannaconne, Donna Yeager, and Sally Cross. An international co-production between the United States and the United Kingdom, the film follows a group of adults responsible for a prank gone wrong on April Fool's Day who are invited to a reunion at their defunct high school where a masked killer awaits inside.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.

Poster © the film's rights holders.