There isn't much you can plan apart from knowing your exits.
You have no idea how many of them there would be, where they would enter, how many different access points they would use, if they know the lay out of the building, if they have placed bombs outside to catch people who manage to escape, will they set a fire to evacuate the building etc.
Bomb training and Fire evacuation is very different and staff are asked to look in their immediate working area, because you are more likely to notice something out of place. In the event of a bomb scare it's not a case of leave immediately, like a fire.