Killer's Moon

Killer's Moon is a 1978 British slasher film written and directed by Alan Birkinshaw, with uncredited dialogue written by his novelist sister, Fay Weldon, and starring Anthony Forrest, Tom Marshall, Jane Hayden, JoAnne Good, Nigel Gregory, David Jackson and Lisa Vanderpump. It follows a group of schoolgirls on a choir trip who are terrorized by four escaped psychiatric patients on LSD while staying in a remote hotel in the Lake District.

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