Don't Deliver Us from Evil(1971)

Original title: Mais ne nous delivrez pas du mal

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.