I think his teachers also have responsibility for the tragedy. The classroom teacher had tried to call him and his mother. He failed to contact him many times. That was just an excuse. And the home room teacher didn't do more on that. How about his colleagues and the principal? Nothing!
It was enough time to take other actions. They could ask the police. They could ask the Child Consultation Center. They didn't do anything they could.
If the teachers more carefully observed their students, this tragedy wouldn't have been happened.
I hope these brutal killers will be arrested soon and the similar victims won't be found in the world.