If a vehicle is totalled by a stolen school bus that was in an unsecured lot with a spare key inside, who is responsible for the damages?

1) The perpetrator is definitely responsible, and could be sued (of course, he might not have income or assets to pay the damages)--anyone who destroys property due to intentionally wrongful actions and/or due to his carelessness as a driver is liable.

2) Whomever was responsible for securing the bus *might* be liable. Generally, people and entities are not responsible for damages flowing from another's criminal acts, unless it can be shown that they were so negligent, or careless, as to greatly contribute to the act occuring. It may be the case that securing a lot and leaving a key in the bus was negligent enough to trigger liability.

Unless you have adequate insurance coverage (in which case, the insurer will pay you; let them then worry about recovering their expense), it would be worthwhile to consult in detail with an attorney about this situatioin.