Majutsu Guild Sousui - Umarekawatte Imasara Yarinaosu Nidome no Gakuin Seikatsu

The Heavenly Visitor and Great Sage, Cedric, is a sorcerer with immense magical prowess, so much so that he has become known as the living legend. In an unfortunate twist of fate, he faces an early demise at the hands of his traitorous subordinates. But Cedric turns this predicament into an opportunity for even further growth. Having felt the limits of his power, he had been studying the secret of rebirth to begin anew as a newborn human. However, when he puts his research into practice, he finds himself in the body of Alphred, a weak boy who is being picked on at school! Frustrated with his circumstances, he offers up his life to summon an avenger in Cedric, setting the stage for an epic revenge saga of the greatest sage!

Source: K Manga