I've always told my kids that they should GET OUT BY ANY MEANS if something would happen at school. Even jumping out a 2 story window and breaking something is better than dead.
Basically once an intruder is reported, an announcement is made that all the staff, faculty and students know means, find a place to hide. Teachers immediately go to the classroom door and get any child in the hallway, into their classroom. We than lock the door from the inside, and have the students get on the floor near an interior wall, (not visable from the door window)We turn out the lights and the kids sit silently.