A warning: This movie is all about the journey. There is no payoff in the end so don't stick around hoping for a nice tidy conclusion. Instead, you will feel the same frustration as the teacher does which I think is exactly what the director was going for in this movie.
Having that out of the way, the basic premise is a school dealing with a thief. The main character (a teacher) gets strong evidence but not conclusive proof as to who the thief is. Mayhem then ensues as the teacher's life spirals out of control.