Students are led out of school as members of the Fountain Police Department take part in an Active Shooter Response Training exercise at Fountain Middle School in Fountain, Colo., on June 9, 2017. Since 2012, a total of 73 students have been killed in school shootings with at least four victims shot and two victims killed, according to research by James Alan Fox, a criminologist at Northeastern University who studies mass killings. (Dougal Brownlie/The Gazette via AP, File)
