The parents apparently knew their son was capable of such a horrific act. Knowing that, they bought him a gun anyway. They taught him to use it. They urged him to not get caught looking up ammo on his phone. They didn't lock up the gun. They didn't warn the school. They didn't search his backpack. They refused to take him home.
The students who stayed home should not be blamed. The school was aware of the rumors and had assured everyone that it was safe. They posted a letter:
"We are aware of the numerous rumors that have been circulating throughout our building this week. Student interpretations of social media posts and false information have exacerbated the overall concern. We want our parents and students to know there has been no threat to our building nor our students."
Some Students Stayed Home Ahead of Oxford School Shooting Due to Online Threats (newsweek.com)
Those that chose to stay home out of fear and lack of trust in the school should not be blamed in any way.
Is the school liable? They obviously knew there was a problem. I'm not sure if they knew EC was the source of the rumors. But they certainly should have suspected when they found his disturbing drawing.
MOO