posted by HereticTroubled people deciding to go out with a blaze of glory would be stopped because someone else at the school has a gun? Not likely. I highly doubt that people who're planning to take a gun into a crowded building/venue are expecting to get away after causing some carnage -- more like an "I'm gonna kill until I get stopped!" mindset. The benefit to having qualified armed people is to hopefully reduce the amount of carnage that a shooter can cause.
It won't stop them but there is a good chance it will lessen the damage.
A suicidal active shooter will keep shooting until they are dropped. If one has to wait for the police that is already shown to be 5-10 minutes easily.
If said student runs into a few teachers with guns, they probably are dropped before the cops get there. Let's say lets use brutally honest kill rates of 1 or 2 a minute, if they are taken down in 3 minutes by a teacher rather than 10 when the police get there that lessens the body count from almost 20 down to 5 or 6.
I don't think anyone is saying it will stop them from showing up, but it can help it from being a massacre.