These are two different sorts of figures being discussed. The Everytown data aren't too far off from what is really going on. There are a lot of shootings at schools (in or on property). The CNN story basically covers the spectacular interest school shootings. CNN recognized this, though I think some of y'all missed it.
All CNN is saying is that there were 15 like what happened in Oregon, and NOT that 80% of the claimed shootings didn't happen. They haven't really corrected a number, but instead highlighted the 15 really sensational ones out of the bunch, the most horrific, those that cause the most fear in American society. A shooting at school is still a school shooting, though it may be typical interpersonal violence as opposed to the helter skelter rampage and often suicidal sort of school shoot like Columbine or Sandyhook.
Of course, any data used that were from events taking place off of campus do not count, but in looking at the critiques of the data, there are only a few of those to be discounted. The rest of the issues seem to be focused on that motivation and ramification differences.
Everytown was fairly specific in stating what their data did entail...
http://everytown.org/article/schoolshootings/
Data: Incidents were classified as school shootings when a firearm was discharged inside a school building or on school or campus grounds, as documented in publicly reported news accounts. This includes assaults, homicides, suicides, and accidental shootings. Incidents in which guns were brought into schools but not fired there, or were fired off school grounds after having been possessed in schools, were not included. This list includes incidents meeting the above criteria that were brought to our attention after our School Shootings Analysis was issued on February 10, 2014. Incidents were identified through media reports, so this is likely an undercount of the true total.
Are there problems with the data? Sure, some didn't occur in or on school grounds. For example, number 8 just occurred "near Morehouse College" and not actually at it.
http://www.myfoxatlanta.com/story/22648057/suspect-arrested-in-fatal-shooting-near-morehouse-college
#15 happened off campus ...
http://www.wvec.com/news/escu-221819011.html
#52 happened on campus but after hours during a pickup game of basketball. So this was a non-school activity during non-school hours, where the activity and the people were in no way monitored or covered by school parameters.
http://www.cleveland.com/metro/index.ssf/2014/02/police_two_boys_charged_in_bru.html
So there are some problems with the data, no doubt, but there are certainly a lot more shootings that occur on school premises during school hours/activities than just the 15 highlighted by CNN.
So some of the events don't exactly fit Everytown's criteria (off campus incidents), but that is only a small portion of the data.