Satan's School for Girls (1973) on Netflix

Plot summary

Martha Sayers is found hanged in the living room at Elizabeth's lakeside house in Los Angeles, and the police rule the death an unmotivated suicide. Elizabeth refuses that explanation. She visits the exclusive academy Martha had attended, against a warning from Martha's roommate, Lucy Dembrow, and registers there under the name Elizabeth Morgan. Roberta Lockhart, Debbie Jones and Jody Keller are among the classmates who welcome her. The feared headmistress, Mrs. Jessica Williams, is troubled by the newcomer's influence. Elizabeth notices several strange occurrences but postpones her questions. Debbie loses her composure in class, and a painting of Martha interests Elizabeth.