Boku no Watashi no Yusha Gaku

Summary

A mysterious "transfer student" shows up in a peaceful classroom at Holy Lance Academy. Their teacher announces that he is quitting because he won the summer jumbo lottery and continuing to teach would be stupid, much to the disgust and surprise of the students. It's suddenly revealed that the weird new student is actually their new homeroom teacher, Haganeno Ken, who wastes no time in saying that he is a hero.