I don't think you have proof of this claim. I am fairly confident that most mass shootings do not occur in gun free zones. They most often occur in non-public areas and garner little press coverage. They actually happen quite regularly and don't even get much play in the media or here.
What you're referring to would be domestic violence. In the small town in Alaska that I used to live, there was a "mass murder" (first of it's kind there) a few years ago. 18ish Y.O. kid snapped, and took a kitchen knife to his family. Killed three, wounded another. Even though the number would make it a mass murder, it was labeled as domestic violence.
What I'm talking about is public mass shootings, where the victims and shooter have no more than an acquaintance status (and more likely, no relationship at all). It's a tough subject. I would never say that mass killings only occur where guns aren't allowed. It's simply not true. But many do. The Norway killing, for example, is a great example. He chose that target mainly because he knew there would be no resistance. The man who shot up the Jewish Community Center in 1999 chose that target because he knew there would be little to no resistance. He had two or three other targets in mind, but was afraid people might have a means of fighting back, or there was security. The wikipedia page on the shooting
says "...security measures presented too much of a problem."
I guess my real point isn't that mass shootings only occur in places that don't allow guns. They do happen with frequency in those areas, either coincidentally, or purposely. My real point is the point most of us are aware of...no gun areas aren't safer, and are in actuality more dangerous than areas where guns are allowed.
This is the same reason Germany didn't invade Switzerland during WWII, and Japan didn't invade the mainland US after Pearl Harbor (they probably could have done it with the Pacific fleet decimated the way it was). They knew citizens of these countries had weapons in their homes and the last thing they wanted to do was get into an insurgent war.
Harris and Klebold and numerous other school shooters who were students shot up their own schools, not somebody else's school.
Something to think about and consider (since there's no way to tell). Would those two monsters have shot up their school if there had been a policy that allowed teachers and administrators with carry permits to carry on the premises?
Personally, I'd like to think not. Keep in mind, they cowardly killed themselves when they knew they were about to be taken. It's proven time and time again (yes, there are always exceptions...I know) that as soon as crazy people meet resistance, one of two things happen.
1) They stop what they're doing and surrender/take their own life
2) They focus their attention on the resistance, allowing innocents to get out of the way.
Either way, innocent lives are saved.