Alex get yourself to the school nurse ASAP!! I do understand the money issue but you need PROFESSIONAL medical treatment!!!! Its not about not trusting your parents or doubt, but this is not a injury to deal with, without a doctor. Because you are a minor many paths open to a legal adult require your parents approval. However if you talk to the school nurse calmly and seriously then he/she should help. If not.

Then take yourself to the hospital and insist on seeing a doctor or your regular doctor tell them you can not get your parents to help you get medical care. You are close to a legal adult age and this is a very adult decision. Your parents fear of making a financial blunder is clouding their perspective. In this situation you will have to push the issue to get care.