"In this report, 51 averted incidents of school violence, a sample drawn from the Averted School Violence (ASV) database, were analyzed to help further our understanding of averted school attacks. The ASV project defines an incident of averted school violence as a violent attack planned with or without the use of a firearm that was prevented either before or after the potential perpetrator arrived on school grounds but before any injury or loss of life occurred. The 51 averted attacks do not constitute every incident of school violence that has occurred in the United States since April 20, 1999, nor do they constitute a representative sample. Rather, averted attacks (those that were identified from open sources) were selected based on the amount of information available in open sources and with an effort made to find reports in a wide range of states."
Office of Community Oriented Policing Services: https://www.cops.usdoj.gov/