I couldn't easily find data going back seventy years, but here's some going back to 1970:
https://www.chds.us/sssc/charts-graphs/
Here's some data, sparse as it is, going back further than that:
https://en.wikipedia.org/wiki/List_of_school_shootings_in_the_United_States_(before_2000)#1940s
It's important to note that reliable crime statistics weren't even kept until the 1960s. And even these were less than all-inclusive. The FBI didn't begin compiling its Uniform Crime Report until the mid-1930s. The resulting system had serious flaws, which weren't addressed until the whole system was reformed in the mid-1980s. Most recently, gun enthusiasts have severely limited the ability of the federal government to do adequate research on gun violence, because the results of that research upset their fee fees. Then too, before the advent of social media, most gun violence, including school shootings, remained a local story.
There has been a marked increase in school shootings in the last twenty to thirty years, in large part due to how the US has been flooded with guns in that time, especially since the Supreme Court gutted most sensible gun measures. As you point out, a part of this increase was in the availability of semi-automatic weapons, which have come down in price to the extent that most anyone can buy one. These are the weapons of choice for mass shooters, which because of the Supreme Court and Republican inaction in Congress are now available to most anyone, including adolescents and people with a history of threats and violence.
So now you answer the question: if it's "people" not guns, how is it the US is alone among developed nations in this plague of gun violence? As has been pointed out again and again, all the nations I cited have people, video games, violent movies, issues with mental illness, economic stressors, and every other problem of the modern era you can cite. What they don't have is a culture flooded with hundreds of millions of guns, certainly not the semi-automatic weapons, like the one used by the 14 year old in the most recent school shooting in Georgia.
What makes people in those countries so different from Americans? Are you saying we are a uniquely murderous people?