Have you filed a claim, either with your insurance company or theirs? Once a claim is filed, the insurance company will determine who's fault it is. The claim adjusters will research the accident, get your statements, look at the police report, etc. You can't stop them from filing it on your insurance, just as they can't stop you from filing on theirs. So, file the claim with their insurance first, see what their company says, and then go from there.
Honestly, just reading your comments, if they were stopped due to an accident in front of them, I wouldn't consider it their fault. You should have moved with enough room to clear the bus. Sorry, I know that's not what you wanted to hear!