Mais ne nous delivrez pas du mal/Don't Deliver Us From Evil (1971)
 

Don't Deliver Us from Evil is a 1971 French film directed by Joël Séria. It is loosely based on the Parker–Hulme murder of 1954.

Plot:

Two young convent girls named Anne and Lore (Jeanne Goupil and Catherine Wagener), become friends and decide to spend the summer together. At the convent school they took a vow to do evil. Over summer vacation, left to their own devices, they become more and more demented.

In all my time here, I've never seen anyone mention this film-has anyone seen this mean spirited gem?