No one has said that.
You keep saying that by continually using "excessive" in the way you are doing. Once someone starts an attack, punching someone else about the head and body, the victim has a right to defend themselves with force that may extend to lethality without being excessive:
"When a person, being without fault, is in a place where he has a right to be, is violently assaulted, he may, without retreating, repel by force, and if, in the reasonable exercise of his right of self defense, his assailant is killed, he is justiciable."
Runyan v. State (57 Ind. 80)
This kid, to our knowledge, was not doing anything to place himself at fault. To our knowledge he was in a place where he had a right to be. He was violently assaulted. He repelled his attacker by force. If the assailant was killed, it would have been legally defensible. He has the legally recognized
right* to defend himself in the manner he did. He does not have a responsibility to the attacker to respond in a way to minimize the risk of harm to the attacker.
The only confounding factor is that we're dealing with kids in a school system, and where the attacker would have been arrested and the victim left alone as an adult, in the ********** up school system they both were punished.
eta:* at least in the US, in other countries, YMMV