Hmmm...
What is your point? I mean that tells us about nothing as far as gun violence.. nor as far as a possible change in it then vs now.
Mass shootings at schools is defined in your statistic as a "shooting at a school which killed 4+ people"..... So 10 kids could've been shot in class in 1971 but if they didn't die, it wouldn't count as a mass shooting......And on the other end of the spectrum, 1 kid could be shot and killed every single day in school in 2022 and that also wouldn't count in that statistic.