ENDRO!

ENDRO! summary:

In the Kingdom of Lapanesta, a hero named Yulia "Yusha" Chardiet and her party of friends manage to defeat the Demon Lord. However, their sealing spell goes wrong and the girls inadvertently send the Demon Lord back in time to before Yusha became a hero. Reverted to the size of a small girl, the Demon Lord, now known as Mao, becomes a teacher at Yusha's Adventure School with the aim of expelling her so that she'll never become a hero in the future.