Guys, if it gets to the point where an armed teacher actually has to intervene, then odds are we've failed already, as that means that PREVENTION has failed, and odds are a few die (at least) until an armed teacher gets involved.
What we need to do is PREVENT this shit from happening altogether, via EFFICIENT use of the NICS system, POLICE doing their damn jobs, and CIVILIANS reporting suspicious activity (like the kid building pipe bombs in your neighbors garage).
The act of placing armed teachers and guards at schools is, in my opinion, entirely a preventative measure. If it gets to the point where intervention is needed, odds are at least a few will die before the person is stopped.