Developmentally-appropriate active shooter response training for schools is supported by evidence.

Official Report From the Attack on Columbine High School

FBI Report: Active Shooter Incidents in the United States in 2018

Rampage: The Social Roots of School Shootings. By Katherine S Newman.
The Survivor's Club: The Secrets and Science that Could Save Your Life. By Ben Sherwood.
Creating the Peaceable School: Program Guide : A Comprehensive Program for Teaching Conflict Resolution. By Richard J. Bodine, Donna K. Crawford, Fred Schrumpf
Innocent Targets: When Terrorism Comes to School. By Michael Dorn and Chris Dorn.