Immediately call a lockdown and follow the same process used in your drills. Utilize the intercom or communication system to announce the lockdown school-wide.
Federal Emergency Management Agency Basic Lockdown Protocol - FEMA has a simple one pager that can be used for basic lockdown protocol.
Department of Homeland Security Active Shooter Tips - While not every lockdown will be the result of an active shooter, there are some guidelines on actions in the event of an active shooter situation.
Active Shooter Response video - This video was created by the City of Houston and covers 6 minutes of tips on what to do if faced with an active shooter. This is a good video to use with staff.