As part of its mission to promote school safety and security across the nation, CISA sponsored the pilot School Security Simulation Experiment (SIMEX) in collaboration with MITRE (a non-for-profit research and development organization) and George Mason University (GMU). This two-week-long SIMEX focused on examining current security procedures and technologies with the goal of producing data-informed recommendations to improve both physical and operational security in K-12 schools—primarily against active assailants. Using virtual reality technology, this SIMEX specifically focused on evaluating three main variables and their effect on the outcome of an active shooter event: utilization of School Resource Officers (SRO), door locking policies, and staff capabilities for lockdown initiation. The key findings and takeaways from this SIMEX are detailed in an After-Action Report (AAR) and were developed based on quantitative and qualitative data analysis collected throughout the event. The goal of these findings is to provide data-informed conclusions and recommendations regarding physical and operational security in K-12 schools in the hopes of informing policy and operational decisions at the state, district, and school level.