Nishuume Cheat no Tensei Madoushi - Saikyou ga 1000-nengo ni Tensei Shitara, Jinsei Yoyuu Sugimashita

Troubled by his overwhelming strength, the mage too strong for the world chooses to give up on his current life and decides to be reborn a thousand years into the future. However, he is greeted by a world in which the knowledge and usage of magic have regressed drastically, leaving his ambitions of finding someone stronger than him shattered. Going by his new name Kurt, the mage encounters a girl called Lala, who encourages him to enroll at the royal capital's magic academy, easily achieved as he maintains the powers he had accumulated in his past life. Meeting new friends and foes there, the world he finds himself in has new challenges in store, but this second life may just offer the enjoyment the cheat mage Kurt desperately pursues! Source: K Manga