The Substitute 2: School's Out (1998)

A mercenary with a chip on his shoulder, Karl Thomasson (Treat Williams) poses as a teacher at an inner-city school to exact revenge on the gang members who murdered his brother. As Thomasson investigates the killing and tries to keep his estranged niece, Anya (Susan Pratt), safe, he begins to realize how dysfunctional and violent the urban school environment has become. Even with support from fellow soldier Joey Six (Angel David), Thomasson is seriously outnumbered as he seeks vengeance.