Hitorijime My Hero Vol 1

Hitorijime My Hero by Memeco Arii. A Yaoi/BL manga rated Older Teen, published in English by Kodansha.

Masahiro, a cynical teen stuck running errands for a local gang, is rescued by Kousuke, a legendary tough guy, who turns out to be his new math teacher. A sweet, slow-burn BL romance.