Been looking for this movie for 30 years. I could not even find a bootleg all these years.
I was always fascinated by the plot description in Leonard Maltin's review in his movie guide:
“Unman, Wittering and Zigo (1971-British) C-102m. D: John Mackenzie. David Hemmings, Carolyn Seymour, Douglas Wilmer, Hamilton Dyce, Anthony Haygarth, Donald Gee. Nifty little sleeper with Hemmings as new boys’ school teacher who is promptly informed by his class that they murdered his predecessor, and that he’d better stay in line . . . or else. Creepy, chilling mystery, loaded with twists and stylishly photographed by Geoffrey Unsworth. Title refers to the last three names on the roll . . . but Zigo is always absent. ”