Nunsploitation Japan-Style: School of the Holy Beast (1974).

Sez Wikipedia: A young woman (Yumi Takigawa) becomes a nun at the Sacred Heart Convent to find out what happened to her mother years earlier. She encounters a lesbian mother superior, lecherous archbishops, and uncovers many dark secrets. The convent also practices brutal discipline and encourages masochistic rituals such as self-flagellation. In one scene, two nuns are forced to strip to the waist and whip each other severely with heavy floggers. Later, Takigawa is tortured and whipped by a group of nuns armed with rose-thorns. (Thanks to Listener Brian Gyss.)