I was a passenger in a semi-public bus when we got into an accident, who is responsible for my medical expenses?

You may have claims against the bus company and the other driver involved in the crash, depending on how the accident occurred.  Typically in Massachusetts, there is no fault insurance for Personal Injury protection that covers your medical bills.  This would be held by the bus company.  The PIP insurance covers the first $2000.00 of bills if you have health insurance and $8000.00 if you do not. 

You should consult with a personal injury attorney to make sure you recover the full amount you are entitled to.