Thug Teacher

Na Dae Gi was the leader of a student gang and had power over all high schools in his area. He declares the gang's disbandment, goes to a third-degree College of Education to become a teacher and finds a job in Kangho High, which has a reputation for being highly populated with troublemakers.
The student gangs at Kangho High, who all the other teachers have given up on, start a fight with Dae Gi, who prepares a lesson of his own, and so begins the war between the gangster teacher and the student gang at Kangho High.
(Source: HanCinema)