Demoted to a Teacher, the Strongest Sage Raises an Unbeatable Class is a Manga/Manhwa/Manhua in english language, Shounen series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

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...