Mass shooting is specially a problem in the United States (and a relatively recent one at that). You can't blame it on the modern schooling system since it has been ubiquitous since the industrial revolution, and has become the global standard the past century or so.
I think the problem goes deeper, and it has to do with the loss of community that technology and development brings about. This is exacerbated in North European cultures, like that of the United States, due to the inherently individualistic and productivity oriented outlook. Mentally vulnerable individuals suffer in their isolated bubbles, and end up breaking down, and sometimes lashing out.
I recommend you give the book Tribe: on Homecoming and Belonging a look. It tackles the question of why many people, specially men, specially in developed countries, and specially in the United States, miss war, and often find it difficult to readjust in modern society. The book often reaches in its explanations, but that is expected since it's written by a journalist and not a sociologist. The thesis it advances for the astronomical rates of PTSD for US war vets, even those who never saw combat, applies to problems like mass shootings and other extreme anti-social behavior. It's a good quick read.