Having attended an inner city high school, I will attest that while some of this was exaggerated, similar things do happen. I think the director took a lot of creative license and pushed things to a bit of an extreme with Garfield and Cesar's characters, but it only served to improve the quality of the film.
This is less "Dangerous Minds" or "The Substitute" than it is "Taxi Driver". Samuel L. Jackson does a superb job with his character Trevor Garfield, a man of deep moral convinctions and idealism who crumbles psychologically throughout the film. The way it is shot, along with the ominous soundtrack, creates an atmosphere of palpable doom and chaos. Garfield's speech to a fellow teacher who is beginning to realize the odd connection between the disappearance of troublesome students and his relation to them is really disturbing. The director should have worked more on the "teacher snapping" bit and had it a little less covert, but overall I would say this movie falls into cult classic, if not classic, range. The darkness is unforgettable, and the film does raise some relevant issues as to how people with values interact with those who have none. There is a certain flavor to this movie, somewhat inarticulate, that for me makes it worthy of the most lavish praise. This is no uplifting, Sidney Poitier film of redemption. It is simultaneously a vigilante film and a comment on conscious man and his place in the world. This is a must own, for Jackson's performance and the powerhouse ending.