It was not put together by the NRA.
When you check out a website and try to figure out the veracity of the information provided there, be sure to look at who put the information together. Do they have a personal or organizational ax to grind?
For this particular site, there isn't an organizational bias that I can see.
This covers decades of data, so you can see that school shootings are not new. I like that map, but also the data sets and charts they included. Be sure and check out those. You will note, for instance, that the majority of school shootings were with a handgun.
Luke Meyers, Mr. Aloha, shared the link above.