Instead of, or in addition to, having armed guards at every school, which is not fail-safe (Columbine had two armed guards), why not just issue bulletproof vests to every child and require that they be worn during school hours? That'd be a lot cheaper.
If we want to guarantee that a child will never be harmed at a school, eliminate all schools. Home schooling will guarantee no mass murders at school.
Yes, this was a tragedy, but we must not overreact.