When a student writes the police code for homicide, 187, inside a textbook owned by teacher Trevor Garfield (Samuel L. Jackson), he feels threatened. The principal dismisses the incident, but, soon after, Trevor is stabbed by the same student. Fifteen months later, a physically and emotionally scarred Trevor relocates to California and takes up substitute teaching. To his dismay, his new school is as full of dangerously undisciplined students as the last one, driving Trevor over the edge.
When schools become war zones and both sides start taking casualties, what then?