Majutsu Guild Sousui ~Umarekawatte Imasara Yarinaosu Nidome No Gakuin Seikatsu~ summary:

Cedric, the Great Sage of Heaven. He was called a living legend because of his overwhelming magical power until he died an unlucky death due to the betrayal of his subordinate. Feeling the limits of his power, Cedric plans to start over by reincarnating as a child. However, he gets reincarnated in the body of a bullied student! For the young man who went as far as to give his life to call Cedric, a revenge drama of the strongest great sage begins!