La residencia (1969)

Lilli Palmer owns and runs a school for wayward girls in France. Her absolute discipline has fostered a social order among the girls with rampant sex, lesbianism and torture the norm. Palmer also has an adolescent son (Moulder Brown) she tries to keep isolated from the young women lest he be tainted by sexual relations; She explains that he must wait for a girl "just like his mother". Meanwhile, girls are "running away" (being murdered) one by one, with their corpses and any evidence of their outcome not to be found. Beware cut versions; the film was shown back in the 70's on network TV chopped to only 76 minutes!