Majutsu Guild Sousui - Umarekawatte Imasara Yarinaosu Nidome no Gakuin Seikatsu

The Heavenly Visitor and Great Sage, Cedric, was a sorcerer of unparalleled magical power, earning him the title of living legend. Yet, in an ironic twist, he meets an untimely end at the hands of his disloyal subordinates. But Cedric transforms this adversity into a chance for unprecedented advancement. Realizing the boundaries of his strength, he delved into the mysteries of rebirth, aiming to start anew as a human infant. However, when he puts his studies into practice, he unexpectedly finds himself inhabiting the body of Alphred, a frail boy who is constantly bullied at school! Frustrated by his new reality, Cedric decides to offer up his life to summon an avenger, setting the stage for an epic tale of revenge by the greatest sage! Source: K Manga