Many schools require you to provide a good credit report prior to matriculation. This is the case with most if not all private school. The reason being that you will need to take out private loans to cover your medical school cost. FAFSA provides roughly 30 some thousand in Federal loans, but again you still need about 15 thousand to cover your total expenses (roughly $45,000) at most Private schools. So the Financial Aid Office wants to make sure that you WILL qualif for these private loans(All of which check your credit.) However , most schools don't ask for the credit report till aftergranting you admission. If you have bad credit, then you need to somhow prove that you will be able to come up with that remaining 15 grand somehow, every year for the next four years. You can show that you will get this money from parents, or your savings etc.
Hope that helped....