Demoted to a Teacher, the Strongest Sage Raises an Unbeatable Class(2020)

Sasen Sareta Saikyo Kenja Kyoushi ni natte Muteki no Class wo Tsukuriageru

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!)