Mark L. Lester said the film was inspired by several true stories of school violence he had read about in papers. In particular was a story of a teacher who brought a gun to class to control his students - the scene ended up in the film.
Roddy McDowall actually drove the car in the scene where Corrigan goes crazy and attempts to run down Stegman's gang. The camera was mounted on the hood as McDowall drove the vehicle crazily down the street.
Much of the graffiti the art director put on the walls of the school for filming was extremely difficult to remove. It caused some trouble when the actual students of the high school used in the film returned to see all of the profanity left on the walls. Ultimately it was all removed though.
The trivia items below may give away important plot points.
Real dead animals were used for the scene where the school finds Corrigan's lab animals killed. The smell of the room they were placed in was so overwhelming that most of the actors wouldn't enter the room.
Director Mark L. Lester felt the scene where Corrigan holds the class at gunpoint was so pivotal to the film that he set aside two days to film that scene alone. Lester says it remains his favorite scene in the film.
In the final scene, where Stegman is hanging on a rope above the orchestra, he says he will change his ways and Andy offers his hand. In one version, Stegman looks to cut himself loose from Norris and falls. However, during a screening, a distributor wanted to see Stegman get his. So after the screening, Mark L. Lester and Perry King did a quick shot of Norris reacting to Stegman's bold move with King looking back in disgust and punching at the camera.