Don't Deliver Us from Evil (1971)

Anne and Lore, neighbors and best friends, barely into their teens, board at a convent school where they have taken a vow to sin and to serve Satan. Anne keeps a secret diary, they read a salacious novel, they get a classmate in trouble, they spy on the nuns, they set aside their communion wafers; they make a pact of devotion. Summer vacation starts: Anne's parents leave her alone with the servants for two months at the family château. She and Lore are free to make mischief. They are cruel as well and play games of seduction. As summer ends and fall term begins, things come to a head.