It's a good thing he stood up the the bully, and it must have been pretty bad to get to the point where you take a knife to school just to defence yourself, but killing another living being, is not a good thing, and just because you're being bullied, it doesn't justify it imo.
Besides that, the 14 year old kid has to grow up knowing he has taken another mans life, people who server in the armed forces sometimes can't handle this, and have breakdowns, so imagine what it's going to be like for someone that young.