Akashic Records of Bastard Magic Instructor

In the "action fantasy" story, Glenn is a part-time teacher at a magic school who is inclined to write "self-study" on the blackboard and then take a nap. One of his students, Sistine, gets angry and challenges him to a duel and he is easily defeated. However, when a terrible incident threatens the school, Glenn shows intense dedication to protecting his students.

(Source: Anime News Network)