Not having access to a gun is absolutely a factor. Teens do not have fully formed brains. Even the best of them don't think clearly all of the time. The fact that many places in the US don't even require guns to be locked up is crazy. Gun laws make a difference. Here in Canada we have had a very small number of school shootings as compared to the US. Similar with Europe where school shootings are rare. Accessibility is a big problem. Guns are not toys and require a level of responsibility that often does not seem to be respected. And yes, parental responsibility is a big factor too.