I agree you need to arm the teachers not just a cops. A cop can refuse to do his job a teacher must do their job to defend their own life.
That said this is a mental health issue we need to do reviews of every student and start institutionalizing people again. We do that and we will reduce a lot more crime and suffering than just the occasional school massacre.
Israel has this same problem, only with whack-job adult terrorists instead of whack-job teenagers.
They have armed guards at every school entrance, with fully automatic weapons, metal detectors, and all the security of and airport.
Yes, it’s expensive and time consuming, but it works.................