Following the Kamino Ward Incident and U.A's shift to an all-dorm system, Inko Midoriya requests that she meet weekly with one of Izuku's teachers to be kept up-to-date on his studies and especially his wellbeing. All Might, having a lighter courseload than his fellow teachers, and being one of Izuku's closest mentors, volunteers for the job. Spoilers prior to season 3 episode 12.