The FBI has issued a new report (6/2018) on the Pre-Attack Behaviors of Active Shooters in the United States Between 2000 and 2013. While the report doesn’t just focus on school shooters, the report should be read to identify behaviors and allow schools to get rid of incorrect information.
https://www.fbi.gov/file-repository/pre-attack-behaviors-of-active-shooters-in-us-2000-2013.pdf/view