Hi there! This is a very good question, and I'd like to express my opinion.Grades are meant to measure someone's performance, but they are limited to the education system - which is not always resulting in accuracy. And this doesn't apply to music only. People learn skills in different ways and pace. The passion for music goes beyond the simple system of grading, and it definitely doesn't depend on a single professor's opinion - which can be subjective.

I'm trying to say that musical knowledge is a must, but it isn't related to how good your grades are. Remember this: people go to college for connections, not for becoming experts. The part when you start becoming an expert in your field is after you finish college and start going deeper in your desired niche of music. That's how you start learning from others and gain experience.