The adults share just as much blame, as always with bullying. Children are left alone to cope with harassment, abuse, violence and assault that would be instantly dealt with if it involved adults. It's always amazed me how children, because they are children, will be left to endure all of this. Yet in every way they have less ability to cope with it.
Imagine your coworker being able to break your arm or try to drown you and it being laughed off. It wouldn't happen because adults have rights, and even mild verbal abuse would have quick action taken to oppose it. Yet somehow in many places children don't have this right to their own safety and peace in the same way that adults do.
The parents, the school staff and the police neglected their duty of care. Bullying is never invisible even at the verbal stage, but they let it continue for months rather then reacting with consequences. Punishments, suspensions, expulsions. They let it continue to violence up to breaking the poor boy's arm! That there should have seen charges laid, for the students and their parents. But nope. Then this.
We get as much bullying as the adults are willing to allow. Fuckers.