It’s no where near a complete list, as MJ cut off entry it appears at a certain number of people killed.

For example, I remembered one left off the list from here in San Diego county not long ago, which happens to show up in the CNN list of mass shootings at schools:

cnn.com

Only 2 died in that 2001 shooting, though 13 were injured. But it didn’t make the MJ list - which only goes to show that there are very many more mass shootings than what is on the MJ list.