Isekai Tensei shita Moto Kyoushi, "Rinji Kyoushi" toshite Houkai shita Majutsu Gakuen wo Sukuu.

In his past life, Gil was a Japanese schoolteacher. In this life, he's a mage who earned the title of Great Sage in a fantasy world. Unable to refuse a friend's plea, he takes on the role of temporary teacher Geek at a troubled magical academy, setting aside his research.

What awaits him is a chaotic institution where arrogant noble students look down on their teachers, classrooms have descended into anarchy, and dark secrets lurk. From insidious bullying to corrupt faculty, the academy's shadows threaten its students—but Geek intervenes without mercy.

Common sense and authority mean nothing before the unparalleled might of the Great Sage. Armed with knowledge from his past life and extraordinary magic, he uproots the academy's corrupt traditions!

(Source: SB Creative, translated)