Before you could literally order guns from walmart with just your address but school shootings weren't even a thing back then. The problem lies in the education system, bullying results in just a slap on the wrist and the school system is ass overall. If you overhaul education, you'll be killing 2 birds with 1 stone, resulting in far fewer/no school shootings + a better quality of education.
As for civilians with guns being morons, with 42% of US citizens owning guns in 2018 I don't think it's really fair to judge the many by the actions of the few. When you compare Texas with California you'll see that gun homicides are at about the same level despite California having fewer gun owners and stricter gun control laws - laws dont stop criminals.
Focusing the prison system towards rehabilitation will also be beneficial since there will be fewer repeat criminals and most gun homicides are gang-related.