I can see that the problem in here is the security of the school itself. None of this will happen if they better the security. I'm not from US but I have a question, does American schools have high security or having a security guard for the school premises? On my country we have. I think this should serve a lesson to all schools for the betterment of their security.

Have you ever heard of passing by catastrophe? Is this true? https://en.wikipedia.org/wiki/Pass_by_catastrophe. Somehow it's connected to this.