A lot of classroom doors have windows. If this is the case, would have the children line up along the wall where they cannot be seen from the outside of the door.

Once an intruder enters the campus space the campus should be placed on lockdown. Are classroom doors routinely locked?

When I worked for one factory, we had a place assigned in case we had to evacuate the building (fire). Each group had someone assigned to determine if the entire group was there. In case of a fire, you don't really want to send someone back inside to look for someone. It was considered importand to know who was/wasn't outside.

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Luke 22:36