Man how the times have changed. When I was younger, I grew up in a not so friendly part of town, and went to school with a bunch of kids that looked at me different due to my upbringing (inter-racial family). I had been beat on, kicked, smacked around, and often times my only thoughts were of revenge. Sometimes, you have to take matters into your own hands, and if you were never on the receiving end of such bullying, in a similar environment, I don't really think you can give a valid opinion on the matter. There isn't always an easy way out as some posters would like to believe, and it's usually the party getting bullied that ends up suffering the most.
Maybe my views are a little more jaded than the normal poster because of my childhood, but I really can't see anything wrong with this. He acted in self defense, though assuming he most likely was suffering too much from fear, panic, or shock when he stabbed the other kid, and ended up going a little overboard. From accounts from family and friends that have served, it's not uncommon when you're put in a high stress situation, that may already have your body pumping more adrenaline than it knows what to do with, I mean you are fighting for your life, and this kid most likely felt the same way.
Sure the kid will have to suffer with the reminder he took another life, if he's even remorseful, though it sounds like the other kids had it coming, and himself, and possibly his other classmates a favor. My main concern after all that is said and done though, is now that he's gotten away with murder, what's to stop others from emulating his behavior? If anything, he needs to be punished in some way, to be made an example of, for both parties.