World's Strongest Sorcerer Gets Reincarnated summary:

This story goes back to the Azuka period (550 – 710 A.D.). The protagonist, En No Ozunu, was living peacefully deep in the mountains attended by a married couple of demons, Zenki and Goki. He is old now, but this emaciated elderly person is actually the strongest sorcerer in Japan… This was precisely En No Ozunu, a japanese ultimate maverick that confronted not only the government officials from the Capital, but even the gods. At last, that strongest sorcerer also grew old, comprehended his own destiny and performed a reincarnation technique on himself. This is not the end; things have only get started. On the contrary, he got reincarnated in contemporary Japan, and even as a bullied student attending a Boys’ High School?! The curtain for this Japan-inspired action fantasy, with sorcery elements and powers from Buddhism thrown in the mix, has risen!!