Carry out a fire drill
On an evening prior to this activity, the Leaders should have discussed fire evacuation procedure with the Scouts. This works well after the Scouts have been working on their Fire Safety Activity Badge and/or had a visit to the fire station.
Set the Scouts an activity to do, knowing it will be interupted (so don't pick something that can't be safely left!)
Leaders to discretely turn off the lights in the other rooms.
One leader to stand by the lights in the room.
A YL will burst into the room with an anouncement of fire, (EG: "There is a fire in the cellar and we need to evacuate the building.")
At this point the leader near the lights turns them off.
The Scouts should assemble at the agreed assembly point and the PLs should call their patrol together and do a head-count. Scout Leaders to check that this happens and check everyone is present. Return to the Hall and review the activity.
To make this a bit more interesting, stash a Scout out of the way somewhere (or a Leader) and see if the missing person is duly noted!