I've seen this question asked many times, and I agree that it's indeed a very relevant question in the context of the urgency of the current situation. I understand that in this limited time period we are forced to decide between two sub-optimal outcomes: a) solve matters with systemic oppression, and b) potentially bear the responsibility for loss of innocent lives as a consequence of striving to preserve moral integrity.
However, we should be aware of the civilisation-wide scale as well. I would like to point out that discussing on whether teachers should be armed isn't relevant to the origin of the problem, since it assumes that extreme violence at schools must be taken for granted.
In my view, our energy would be put to better use by debating on the very reasons for the occurrence of this problem and on how to solve it on a more fundamental level. In my humble view, it's a question of whether Homo sapiens evolves towards peace by way of individual agency or towards power struggle, either by way of violence or psychological coercion.