Maou Gakuin no Futekigousha: Shijou Saikyou no Maou no Shiso, Tensei shite Shison-tachi no Gakkou e Kayou

Tired of ceaseless war destroying humans, spirits, and demons alike, the Demon King of Tyranny, Anos Voldigoad, reincarnates to restore peace. Two thousand years have passed...but who could have known that his descendants' complacency would cause magic to regress so much? After receiving an invitation to the illustrious Demon King Academy, a school tasked with locating the reincarnated founding ancestor, Anos resolves to enroll, only to discover that his magic is off the charts—literally! With power too great to be measured, the former Demon King is branded a misfit. How will he convince demonkind that the school's founder stands before them? Thus begins a misfit's climb to the top of the demon hierarchy!

(Source: J-Novel Club)