The first officers to arrive on scene will not stop to help the injured. Expect rescue teams to follow initial officers. These rescue teams will treat and remove the injured. Once you have reached a safe location, you likely will be held in that area by law enforcement until the situation is under control and all witnesses have been identified and questioned. Do not leave the area until law enforcement authorities have instructed you to do so.
When you call for help, prepare to give the dispatcher the following information:
If you are not sure if it is safe to come out: just stay calm, stay put, stay quiet, and stay alive.
U.S. Department of Justice Federal Bureau of Investigation Active Shooter Event Quick Reference Guide
https://kysu.edu/administration-governance/president/safety-security/active-shooter-emergency-plan/
Vice President for Student Engagement
University Police Department