1. Run
Leave all personal items behind and watch for danger along the evacuation route (the shooter may be familiar with the routes and maybe waiting for you at the exit point). Leave your cell phone in your pocket. Keep your hands visible at all times so law enforcement can see that you are not hiding anything.
2. Hide
Turn off your cell phone, including vibration mode, and stay quiet. If you attempt to text family or friends to tell them to alert authorities, have a code word previously set up to let them know it is a legitimate emergency and not a hoax. This will also let them know that they should not call you back.
3. Block
Barricade yourself into a secure room if possible. The shooter may be after you, but you can at least try to slow him or her down by preventing them from getting to you quickly. Creating a block or a series of barriers between you and the shooter(s) is key.
Also, consider that everything and anything can become a weapon in this type of situation. Do not worry about the possible harm to the shooter. There is safety in numbers; attack en masse whenever possible.
Law enforcement agents responding to an active shooter call have only one thing on their mind: to stop the violence. If they see someone clutching a large bag or a cell phone (potential triggering device), they will treat that person as suspicious or as a potential threat since they have no idea what the shooter looks like or if the shooter is acting alone.
Remember, law enforcement agents are focused on stopping the violence. When they see people running toward them, they must make a split-second decision to determine whether or not those people are trying to harm them, and they may guess wrong, leading to unnecessary casualties. After the shooter is stopped, law enforcement will then begin to help the victims and provide aid.