I'm sorry, but you're an idiot.
I am 6 feet tall and I have a 13 year old cousin who is just as tall as I am. Don't spout that bullshit about him being a kid, a 14 year old can hold a gun or knife or other deadly object just like anyone else.
My question is why he felt the need to use deadly force, which the story doesn't say.
One requirement of being a cop is to be able to be physically adept... especially against some kid. If the kid had been holding a gun and threatening to kill someone it's another story, but he was running away, scared. At this point the cop should have ran and caught up to him, and cuffed him. No need to use much force, especially not deadly force. At very most, a taser, but even that would be too far in this situation.
I still find it hilarious that cops are involved over a school fight. Back when I was in school, which is not that long ago, the result of a school fight was whoever lost was walking home with a black eye and possibly a nose bleed and whoever won, well, won. Life goes on, no cops involved. If one guy was a bully and bullied people around all the time, eventually people got together and took him down. Again, no need for cops.