DEMOTED TO A TEACHER, THE STRONGEST SAGE RAISES AN UNBEATABLE CLASS

Monsters have long threatened humanity’s existence. To fight against these beings, Keith, the youngest person to ever become a sage, joins a knight order and shows immense success on the battlefield. However, that doesn’t last long, as a quarrel with an incompetent higher-up gets him fired from the order. With nowhere to go, Keith is assigned as a teacher, and with his talent for bringing out the best in his students, his class is about to reach unbelievable heights... (Source: MANGA UP!) — Description from AniList community (anilist.co).