Endro~!

A hero named Yulia and her companions defeat the Demon Lord, but a mistake in their sealing spell causes him to be sent back in time to before Yulia became a hero. As a result, the Demon Lord is transformed into a small girl named Mao, who enrolls as a teacher at Yulia's Adventure School. Mao's goal is to have Yulia expelled, preventing her from ever becoming a hero.