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!
 (Source: Square Enix Manga & Books)
 
Note: The manga was cancelled after the author passed away.

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)

Anos, 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, Anos 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 Anos 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!
 
(Source: Square Enix Manga & Books)

As peace returns to the demon realm, Anos Voldigoad wishes nothing more than to put his reputation as the Demon King of Tyranny to rest and go back to being a misfit at the prestigious Demon King Academy. Unfortunately, any tranquility is fleeting: sinister demons, kings, and deities plot Anos's demise from the shadows.
 
Rumors spread about the "Child of God," a being whose power may rival that of Anos. To uncover the truth and eliminate the potential threat, Anos must journey deep into the land of spirits. However, the spirit world is shrouded in mystery, and it may only be entered after undergoing a series of difficult trials.
 
With unrivaled power and confidence, Anos braces himself to defeat various formidable enemies with grandiose titles. But he—with the assistance of his trusted allies—will barely have to break a sweat as the true Demon King of Tyranny.
 
(Source: MAL Rewrite)

The second cour of the second season of Maou Gakuin no Futekigousha. 
The academy's lessons are back in session with Anos Voldigoad fighting more powerful enemies! 
(Source: Crunchyroll)

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