You can follow the edweek links to look at their data. It gives enough detail on any given incident for you to make your own call about whether it's included for political reasons or actually representative of the type of thing parents might fear.

Mother Jones maintains a database of mass shootings int the U.S. that would include school mass shootings, and they (to their great credit) use non-crazy criteria to determine what to include: https://www.motherjones.com/politics/2012/12/mass-shootings-...