Now in my father's day, they actually carried their shotguns to school and kept them in the lockers to hunt after school (or before). I dont have exact figures but I would strongly suspect that the level of gun ownership (meaning how many households own a gun) has not changed as a percentage of the population. If anything, it may have declined. Yet there were no shootings like this back then.
Only after the clock tower incident in 1966 did they start and even then it was still rare. Then there was a rash of them in the early 80s and more in the 90s (including Columbine). Since 2007, however, there have been over 21 mass shootings of more than 10 people.
What is it about PEOPLE that has changed? it isn't because there are more guns, more powerful guns or that they have easier access to them. For some reason, PEOPLE have decided that it is ok and that their means to significance is to murder a bunch of people. What is it about society that has taught them that is good? What has changed about society? One thing at least since 2007 is the advent of social media.
Some wonder why culture wars are important, this is part of the reason.