Actually, I think "when" is perfectly acceptable, both in English grammar and the intent of the statement. The building has no other use, so "when" the other building burns, it is used.

Also, it conveys a firmer sense of the reality that the primary building is vulnerable and helps remind people to stay aware of the danger and what to do.

So I vote for "when the building burns."

Obviously they failed the drill. If busses are part of the plan, they should be part of the drill.
I my office we are not allowed to keep our coats and backpacks close to our desk for "safety" reasons either. I keep my hat, my vest ( I usually don´t wear a coat) and my backpack there for health reasons anyway. I can get out of my chair, put on vest and hat and grab my backpack in one fluent motion. Noone has the nerve to complain about it. The last fire drill in adverse weather conditions claimed a couple of victims (just a bad cold, but down for a few days anyway).

One lesson I learned from cold-weather fire drills is to have your car keys with you. I was able to get some cold coworkers warmed up that way.