Pretty sure they also count suicides, self-defense shootings, and police-action shootings within 100 miles of a place of education to be a "school shooting".
There was one time that someone found that one of the school shootings they counted was a drug deal gone bad that took place near a school in the middle of the night.
I'm surprised they don't just go ahead and count every shooting by anyone that ever attended a school at some point in their life.