Andrew Norris is the new music teacher at a troubled inner city school. As he arrives on his first day, he meets fellow teacher Terry Corrigan, who is carrying a gun. When Andrew asks about the firearm, Terry assures him he will learn why the protection is necessary. When they enter the school, Andrew is shocked to see everyone scanned by metal detectors and frisked. He spots a student with a knife, but the security guards let the kid go because they are so overworked.
The halls of the school are covered with graffiti. Andrew learns he is expected to patrol the halls as a security guard during his off periods. In his first class, a group of five disruptive students are roughhousing and causing trouble. The leader of the gang is Peter Stegman, the only member of the group who is actually registered in that class. They all eventually walk out, and Andrew discovers the rest of the students actually want to learn, especially Arthur, who plays the trumpet, and Deneen, who plays the clarinet.
As Andrew gets to know the school and the area, he decides that he wants to put together an orchestra with his more advanced students. Peter’s gang sells drugs, run a strip club, and cause all kinds of mayhem. They follow Andrew home and taunt him one night.
At school, Andrew is confronted with more and more evidence of Peter’s crimes. The two grow increasingly at odds. Eventually, after Peter killed Terry’s animals in his lab, Andrew and Peter wind up in a bathroom alone together. Peter throws himself into a mirror and beats himself, claiming that Andrew attacked him. Trying to clear things up, Andrew visits Peter’s mother at home. Frustrated when Peter still plays the victim and his mother will not hear Andrew out, he hotwires Peter’s car and drives it into a wall. During lunch, the gang starts a “food fight” and forces their friend Vinnie to stab Arthur, which he does so and causes him to be sent to a hospital. Vinnie is arrested and held in a youth detention center. Terry is driven insane after the incident with the animals in his lab and is killed after crashing his car when trying to kill Peter and the others.
Andrew’s orchestra is about to give its first concert. As his wife Diane gets ready at home, Peter’s gang breaks into the house and gang-rapes her. One of them takes a Polaroid of her being raped and has it delivered to Andrew on the podium, just as he is about to start the concert. Horrified by the photo, he runs off the podium in pursuit of Peter’s gang. Andrew and the gang chase each other through the school. Andrew kills them off one by one, and finally confronts Peter on the roof. Their last scuffle ends with Peter falling through a skylight and getting strangled to death in the ropes above the stage. His corpse falls into full view of the audience. Andrew is never charged because the police could not find a witness to the crime.