...then just talk to your advisor!
Let's say that Mommy won't let you into her class. Now Mommy's reasons for not letting you enroll are very silly indeed. Who has ever heard of departmental policies? And why would it matter if you didn't learn all the material properly in the first part of the class? Silly Mommy. No.... bad Mommy.
So what is a poor student to do? Go run to your advisor who, if he is a very good, erm, Daddy, he will send Mommy a very firm (and borderline offensive) email (with you cc'd on it!!) outlining what a nice student you are, what potential you have, and just how much you really want to grow up and be a doctor, just like Grandpa.
I just love it when other faculty members try to undermine MY course policies (and authority) under the guise of being student advocates. Please. If they were really going to act in the student's best interests, they would advise the student to find a new course of study rather than fail a SECOND course in my department.