If my 14 year old son hit a boy in the mouth on the school bus, am I liable for the medical bills?

You are technically responsbile for the actions of your minor child. If they have insurance which covers his expenses, then there is no loss for them.  If you are sued, then you should turn it over to the insurance company which maintains your homeowner's policy

Most states have laws which impose liability upon parents for torts committed by their children. In Kansas a parent can be held liable for damages caused by a minor who maliciously or willfully injures a person or damages property. There is a maximum amount of $5000 for which the parent can be held responsible unless the act of the minor results from parental neglect, then there is no dollar limit on the parents' liability.

Most people don't sue over a pop in the mouth on a school bus.  However, because your son is a minor, you are liable for any injuries or damages that he causes if the parent of this child did decide to sue you on behalf of their child.  Most personal injury attorney's won't file a lawsuit for minor damages, but it won't deter them from at least sending you a demand letter.  If you do receive a demand letter for payment, you do want to have a personal injury defense attorney to make sure that you are not being taken advantage of.