The Misfit of Demon King Academy

The Misfit of Demon King Academy: History's Strongest Demon King Reincarnates and Goes to School with His Descendants (魔王学院の不適合者 ～史上最強の魔王の始祖、転生して子孫たちの学校へ通う～, Maōgakuin no Futekigōsha ~Shijo Saikyō no Maou no Shiso, Tenseishite Shison-tachi no Gakkō he Kayō~), or Maou Gakuin is a light novel series written by Shuu and illustrated by Shizuma Yoshinori. It got adapted into manga series by Kayaharuka. An anime adaptation by Silver Link studio is set to air in July 2020.

Plot
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.

Arnos, 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, Arnos 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 Arnos 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!