I've seen that list many times before and was even going to post it myself. I'm guessing you just skimmed over it and did not read the notes.
So let me rephrase, how many school shooting can you count on your hands that you knew about? I'm guessing you won't make it pass your two hands.
I'm not going to count crime infested ghetto area school shootings that involve a 70% dropout and incarceration rate, where one student shoots another over some argument over a girl. Nor gang related violence in those same schools. They will do that in and out of school as much as anywhere else. It's not related to any particlar place or time.
I'm not going to count a depressed kid that goes into the school to commit suicide in the bathroom. Nor someone that purposely shoots someone else in the butt and gives up. Or attempts to murder one single person at school because they don't know were that other person lives and it's convinent... All those things have nothing to do with schools.
What I'm going to do is only count real school shootings that involve someone walking in and trying to kill as many people as possible. And weight that against all the non-school shooting events in relation to the the number of pupils, schools, and days. Guess what happens then?
Data should always be filtered, outliers removed, then normalized or standardized.