As far as I can tell, the school shootings that occur here in Canada -- which are, for the record, much rarer than they are in the US, even if one accounts for population differences -- aren't any bloodier than the ones down south despite our much stricter gun control, which suggests to me that teachers and other faculty not having guns really isn't the problem here.
Plus, the risks of having armed weapons around kids far, FAR outweigh the potential benefits of having a firearm during a school shooting, being that these kinds of tragic events are not anywhere near as common as people seem to think. The chance of an accident occurring (kid gets a hold of gun and accidentally harms him/herself, teacher does something idiotic despite being licensed and trained in using firearms, etc.) is a hell of a lot higher than some nut turning up and opening fire.