The bodycount might not be "spectacular" but the rate of school shootings in the U.S. dwarfs other developed countries. Fatalities 200+ so far. At any rate, you don't know what they were 'estimating' exactly.
> The vast majority of shootings are between criminal gangs - making the guns illegal will not magically make criminals into upstanding citizens.
Criminals retrieve their weapons through underground networks. School shooters do not. They're just students. They've either grabbed what was lying around at home or bought them. Such that, accessibility to school shooters is hampered were it the case that legal distribution were more restricted.
Add to the fact, circulation of guns in the underground is only possible and bloated because of the sheet number of legal guns.