There have been 13 mass school shootings since 1966, not 27 this year, per a very recently posted Reason.com article. The difference is determined by defining the meaning of school shooting. This is a good read because it offers good reference material and offers objective discussion material. This article highlights (unintentionally or intentionally) the issues with the media inflating data to further their argument.
The meat of the article:
Obviously, 13 incidents in the last 56 years is a very different statistic than 27 incidents in the last few months. The two figures are so far apart because they measure separate things. One-off gun incidents are a serious problem in the U.S., and those taking place at schools are no exception. Mass casualty events, on the other hand, constitute less than 1 percent of all gun deaths. Suicides and non–mass-casualty murders—usually carried out with handguns rather than assault rifles—constitute the overwhelming majority of gun crimes.
There Have Been 13 Mass School Shootings Since 1966, Not 27 This Year (reason.com)