Big Magnum Kuroiwa Sensei

Tetsuo Kuroiwa is a teacher newly assigned to a high school ravaged by violence. Kuroiwa is authorized by the Minister of Education to carry a gun, and he wields his beloved Magnum to subdue the delinquents with overwhelming violence. After cleaning up one school, Kuroiwa moves on to the next.

(Source: Mangapedia, translated)

Note: Dates based on tankoubon release.