This one just stuns me with the sadness of it all.
How to Behave in a School Lockdown
"A school lock down is when students, teachers, and faculty are confined to their rooms due to a perceived or real threat. Throughout the school year, it is common for schools in the U.S. and Canada to repeat lock down drills so that students become habituated to a passive response in the face of life threatening danger. Drills can be scary and you might feel threatened, but if you behave properly, you will not get in trouble."
1. If you know anyone who is doing this as a joke, tell a teacher immediately.
2. Lock the door however you can.