In order to ensure the safety of all during a lock down, I tell my students that everyone may not speak, including me. I created a set of silent reminders to use with the class during this time. If a rule is broken, I just hold up the card. I keep the cards in the closet where we hide for lock downs along with a class coloring sheet and crayons in case it is not just a drill. It's in an editable PowerPoint in case you think of any of your own.
The cards include:
Don't Worry! We'll be just fine!
Move so you can't be seen!
Stay away from the door!
Ignore that! (for knocks, announcements or calls from the hall)