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.
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...