Gestures, in love, are incomparably more attractive, effective and valuable than words.
~ Francois Rabelais
I'd say schools and I'll give you a different potential reason.
School shootings have largely been made up recently of people that attend the school and are school age, be they college or high school.
In both you have at times high stress environments mixed with generally emotionally immature people. With college often times yo uhave people coping for the first time with being away from home and on their own in some way shape or form. In both, but more often in high school, you have people that are continually ridiculed and ostracized from their little pocket society. You have multiple areas with large amounts of people together in a planned and organized fashion in which people can plan accordingly for. And in general security is relatively lax in most of these places with high schools tending to have perhaps a single security guard and colleges a few cops to go over the entire place, along with the issues with lack of guns on the grounds, and the fact that many of these that becoming more common in "middle class" and "white colar" invironments where security checks in general aren't occuring as often.
Add all that together and you have a recipe for potential mass shootings.
That said, given the magnitude of how many educational institutions we have in this country along with the millions that attend said institutions and compare that to the relatively low, relatively speaking, number of mass shootings and you'll see that even with the above stated mass shootings are not really "likely" anywhere in the U.S. outside of potential areas of heavy gang presence.
"I am appalled that somebody who is the nominee...would take that kind of position"
"A court took away a presidency"
"...the brother of a man running for president was the governor of the state..."
It's horrifying because Trump is blunt instead of making overt implications.
I want to add that I think the idea that schools are more likely to have a mass shooting because the people there are unarmed is foolish. I don't think that is a significant factor in some one choosing a target for a mass shooting.
Two schools, identical in every conceivable way EXCEPT one has certain teachers licensed to carry weapons whereas the other does not. Which one are you going to attack?