I believe the judge was right with their ruling, that given the circumstance the kid had little to no choice in defending himself. That the bully died is tragic, but entirely his own fault. Saavedra had been the target of Nuno's bullying for a long time, and he constantly tried to avoid and get away from it. Saavedra knew what Nuno and his pals were capable of, and he also knew they kept escalating it.
Saavedra knew a severe beating was going to be taking place because it was being announced, and so he took action to avoid it by getting off the bus several stops early. Unfortunately it did not work, as Nuno and his pals, intent on bullying Saavedra, followed him off of the bus then. He was still intent on avoiding being beaten and getting away when Nuno hit him in the back of the fucking head. Even with that Saavedra tried to get away. Nuno and his goons still came after him, all of them attacking.
Being outnumbered 3 to 1 in a fistfight is pretty bad, especially if the 1 doesn't have any experience fighting (back) and the 3 do. I don't know what some of you are on to think that such a situation is all hunky dory and that he'd at worst escape with a few bruises. Seriously, just what the fuck? That's the kind of situation where, even if the bullies don't intend to, they can seriously injure, cripple for life, or even kill the target of their abuse. I also do not buy the whole "he was too young to have such responsibility" as that's plenty old enough to drive a car by one's self, so unless you're saying any accidents caused by a person of that same age should just be written off as "whoops, he didn't know any better so shouldn't get in any trouble" I expect you to re-evaluate your position.
So, now that hopefully the situation is clear, I'll continue. Outnumbered 3-1, not being able to escape, and wanting to avoid serious injury, Saavedra was left with no other option but to fight back. He first brandished the knife hoping that alone would make them stop, but they kept beating him. He stabbed Nuno, again hoping that would bring an end to the bodily harm being delivered unto him, but it didn't. So, left with no alternatives Saavedra continued to stab until he was no longer being punched. The fact that he stopped at that point and did not continue stabbing afterward the punches were no longer coming speaks a great deal to his self composure. If anything he should be commended for stopping when he did, delivering only the appropriate amount of force needed to ensure his own safety. This is something even well experienced soldiers and cops can have a hard time doing, especially when they've been struck in the head and are in a very desperate situation.
I agree that Nuno's death was tragic, but Nuno brought it on himself. He could have stopped at any point. He could have decided to not beat up Saavedra. He could have skipped the fight when Saavedra got off the bus early. He could have stopped when Saavedra tried to run away after being punched in the back of the head. He could have stopped when Saavedra pulled out a knife. He could have stopped when he got stabbed. He could have stopped even after multiple stabs. That he chose not too is entirely upon Nuno himself.