Mass shooting is defined as an incident in which 4 or more victims, not including the suspect, are killed.
There are hundreds of school shootings that have taken place across the U.S, but 11 can be classified as mass shootings.
Here is the list of them, in chronological order.
March 21, 2005
October 2, 2006
April 16, 2007
February 14, 2008
April 2, 2012
December 14, 2012
June 7, 2013
October 24, 2014
October 1, 2015
February 14, 2018
May 18, 2018