Shouldn't the comparison be made against school shootings world wide? Or at least be normalized for overall crime rate or something?
The numbers are so tiny that comparisons are pretty much irrelevant. Since 1996 there have been just 8 mass k-12 shootings (incidents involving 4 or more school deaths, excluding the assailant), in a nation of 330 million people. That's so far into small numbers territory that any comparisons are guaranteed to be overwhelmed by noise.
Edit: In a comment below, this article was linked https://qz.com/37015/how-school-killings-in-the-us-stack-up-.... Indeed, on this scale we see a very different picture!