Japanese high school bullying movie?

I'm really not even sure if it's Japanese, Asian of some sort maybe. Anyways all I remember was at the end, this boy who was getting bullied and the bully were walking up these stairs at a high school to get to the roof so they can fight. While walking up the stairs, the boy who was bullied pulls out nun chucks and takes a cheap shot at the bully in the back. He did it because someone had told him that whomever makes the first blow in a fight will win. He ends up beating the bully up and the bullies friends. Hope you can help me