Jonathan Shale is a Vietnam War veteran and mercenary. After a botched covert operation in Cuba in which three men from his platoon were killed, he returns home to Miami. He surprises his girlfriend, Jane Hetzko, at her apartment; she welcomes him warmly. Hetzko is a schoolteacher at inner-city Columbus High School, an institution with a considerable gang problem.

https://en.wikipedia.org/wiki/The_Substitute

• This movie: https://www.youtube.com/watch?v=35VTbozkY-o
• The sequel: https://www.youtube.com/watch?v=bhrlXhyH21o

There are a couple of cuss words blanked out the in the first couple minutes, but whoever was doing that got bored and quit doing it after a while. Also, the youtube comments say some of the violence was edited down.