The most dangerous thing about school used to be the students.

The Substitute (1996) - Shale, a battle-scarred mercenary, finds that the war has moved closer to home when his girlfriend is attacked by a ruthless band of teenagers. Determined to root out this menace, Shale goes undercover at a local high school. Posing as a teacher, he and his team of mercenaries set a trap to catch the ringleaders in this explosive thriller.
The Substitute 2: School's Out (1998) - When teacher Randall Thomasson is killed during a carjacking, an unexpected visitor shows up at his funeral - his estranged brother, Karl Thomasson. An ex-Green Baret turned mercenary, Karl sings on as a new substitute teacher in order to investigate Randall's murder. With his military training, he can easily dispense with knife-wielding students. But when he traces the bullet that killed his brother back to someone within school, Karl is forced to break out the big guns.
The Substitute 3: Winner Takes All (1999) - The Substitute goes undercover once again to bring justice to the classroom. The university football team is deep into an underworld of drugs and blackmail and are using their muscle off the playing field as well. In this action-packed sequel, its up to The Substitute and his crew of mercenaries to teach these students a lesson they will never forget.
The Substitute 4: Failure Is Not An Option (2000) - When a clandestine gang of neo-Nazi students threaten an elite military school, undercover cop Karl Thomasson poses as a teacher and plunges into a deadly cat-and-mouse game of shifting alliances and murderous treachery in a one-man war to prevent and academic apocalypse!