Majutsu Guild Sousui: Umarekawatte Imasara Yarinaosu Nidome no Gakuin Seikatsu

A powerful sorcerer known as Cedric is betrayed and killed by his subordinates. Before his death, he studied the secret of rebirth to start over as a human. Upon being reborn, he finds himself in the body of Alphred, a weak student who is bullied at school. Displeased with his new situation, he sacrifices his life to summon an avenger named Cedric, initiating a story of revenge for the legendary sage.