How has the number of people killed in non-gang, non-suicide shootings at K-12 schools changed over the last 20 years?

UPDATED INFORMATION is available here..

UPDATE: Problems with the Bloomberg claims on school shootings is available here.

(Click on the figure to make it larger.)
Using data from the National School Safety Center, Slate, and Michael Bloomberg shows a significant drop in firearms deaths over the last 20 years. The National School Safety Center already has done a good job of noting whether the shootings involved gangs and whether they occurred on or near school property. Bloomberg and Slate were nowhere near as careful, and required a systematic search of news stories.