As tempting as it may be, don't barge into the school and demand justice. Teachers and administrators will react to your behavior, not your message. Instead, call to arrange a formal meeting. Participants will view you with respect and be more receptive to your concerns, and they will understand that you are very serious about the matter.
Staying calm rather than blowing a gasket is also better for your child. When you overreact to your child's problems, he may start to feel responsible for your discomfort and assume the caretaker role, putting aside his worries to ease yours.