Maou to Ryuuou ni Sodaterareta Shounen wa Gakuen Seikatsu wo Musou suru you desu

A boy named Lucius gets trapped in a distorted space created by the ancient hero. What he finds there is the Demon King and the Dragon King, who should have been defeated by the hero 300 years ago. Lucius befriends the Demon King and the Dragon King, and is trained by them, going from a weak child to a legendary warrior with the power of both demons and dragons.

Using his power, Lucius escaped from the strange place and was accepted into the magic academy where the hero's descendants live. And now he will have to face many challenges to free the Demon King and Dragon King.

(Source: Shousetsuka ni Narou, translated)