The Misfit of Demon King Academy (4 Book Series)
From Book 1: What if you were the strongest demon king in history...but nobody believed you? A light-hearted action rom-com series wherein the devil is definitely not getting his due.
Anoth, the Demon King of Tyranny, has defeated humans, spirits, and gods alike. But even demon kings get tired of all the fighting sometimes! Hoping for a more peaceful life, Anoth decides to reincarnate himself. When he wakes two thousand years later, though, he finds the world has become too peaceful--his descendants have grown weak and magic is in serious decline. Intending to reclaim his rightful place, he enrolls in Demon King Academy, where he finds that his magical power is off the charts. Literally. And because they can't measure his power, the faculty and other students regard Anoth as a misfit. With the support of Misha, the one student he manages to befriend, the misfit (Demon King) begins his climb up the demon ranks!