Bad Education - Rotten Tomatoes
Alma movie: The Substitute (1996)

Teachers with military experience tend to make for effective disciplinarians in the classroom, but Mr. Smith (Tom Berenger) goes just a tad too far. See, Mr. Smith is actually Jonathan Shale, an ex-mercenary who's gone undercover as a History teacher to bring down the local gang -- headed by Marc "Mr. Jennifer Lopez" Anthony, no less -- that roughed up his girlfriend. Naturally, he decides the best way to accomplish this is to rig the school with surveillance equipment, call in a few war buddies, and engage the "bad kids" in an all out firefight on campus. Naturally, that is, if you're a psycho.