Shouldn't the comparison be made against school shootings world wide? Or at least be normalized for overall crime rate or something?
That's so far into small numbers territory that any comparisons are guaranteed to be overwhelmed by noise.
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)
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!