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.
When making a decision as to whether something is small, large, or tiny one needs a scale. What's the scale here? For example, if the rest of the world combined has had 12 mass k-12 shootings, then American accounts for 40% of the total shootings which is a huge fraction. So in my original question I was trying to find the right scale.
That number seems suspicious; you'd get a very different answer if your criterion was "any gunshot wound at a school", for example. But that definition might be a better match for what people think of as a school shooting.
(e.g. Northern Ireland had a very large number of terrorist attacks where a warning was given allowing evacuation - would they not count as terrorist incidents even if nobody was killed? I suspect not)
Any gunshot would at a school probably includes a very different pattern of behavior. If someone wants to murder (or maim) a specific person, or small group of people, they may carry that out with a gun, because it's expedient, but might switch to a knife if guns aren't available. If someone wants to murder (or maim) a large number of people, guns or explosives are really the only practical means; mass knifings have happened, but are even more rare than mass shootings.
