As it turns out, criminals don't really care where they can legally bring guns and will happily walk past a "NO GUNS ALLOWED HERE!" sign.
When gun grabbing legislation is passed, it only hurts law-abiding people, not criminals. Criminals will still get and use guns whenever they'd like.
[1] http://www.cnn.com/SPECIALS/2000/columbine.cd/Pages/DEPUTIES...
And I think you're missing the point. Absent a total ban on citizens of good standing possessing guns inside (including administrators in this case, or, say an arms locker in the teacher's lounge), the duo wouldn't have had free reign as the police cowered outside (experience shows teachers and administrators are a lot more serious and brave about protecting their charges than the local police were that day).
To take another example, the Colorado theater shooter could have gone to closer or bigger ones, but just happened to pick the one that was so ... severe about being "gun free" it insisted off duty police hired as security guards go unarmed. It was gun free, alright, until he showed up.
One correction to this pattern: the Arizona Congresswoman shooting, where an armed citizen showed up as the shooter was being restrained by people there, but one of them in the line could have been legally armed.
Gun advocates pushing the line "school shootings wouldn't happen if there were armed people on campus" sounds suspiciously similar to the joke about selling elephant repellent. Keep in mind, I'm providing you with examples of things that actually happened while your view of gun-provided safety exists in the hypothetical. I am happy living in an area where gun ownership is scarce and shootings (especially accidental) are scarcer.
[1]http://www.lasvegassun.com/news/2014/jun/22/joseph-wilcox-be...
Heck, the Columbine incident is very rare in having two shooters. Per Wikipedia and my memory this only happens every few decades in the US and no other time at a school, and it's equally rare in the rest of the world:
https://en.wikipedia.org/wiki/List_of_rampage_killers_(Ameri...
https://en.wikipedia.org/wiki/List_of_rampage_killers_(schoo...
https://en.wikipedia.org/wiki/List_of_rampage_killers_(workp...
https://en.wikipedia.org/wiki/List_of_rampage_killers#Religi...
https://en.wikipedia.org/wiki/List_of_familicides_in_the_Uni...
Whereas we know of tens of incidents stopped, or so we suspect since a spree killer/mass murderer who's stopped early by definition doesn't succeed in killing lots of people.
http://en.wikipedia.org/wiki/Mitchell_Johnson_and_Andrew_Gol...
Here's your third: #59 in your first link. Here's your fourth: #67 in your first link.
We know of the incidents stopped, but so many of those in your links ended by the killers' own hand. In almost all of the cases, however, these are mass murderers with guns.