After Retirement, I Became a Transfer Student for Three Big Bosses

Twenty years ago, he trained three S-level bosses and retired to open a hardware store. Twenty years later, the system forced him to disguise himself as an E-class loser and enter Kyushu University of Supernatural Powers, where he became a teacher and student at the same school as three "traitors" who had become principals and professors. He just wanted to live quietly until graduation, but the whole school started to focus on him.