All the best musicians I have known are not just good at music: they are good at life. If you want to be a professional musician, then fine, go for it. However, College--especially as an undergraduate--is about developing "the whole man." You cannot use 'music' as an excuse for bad grades, because most trumpet teachers at colleges see poor grades as a poor work ethic. It's usually true, too. If you're willing to put hours in on your trumpet, I don't see why you wouldn't be willing to put in enough effort to get a B or an A in your classes. It takes maybe a 1/2 hour to 1 hour a day, depending on your workload. I remember that, during my senior year of high school, I had a 4-hour-a-day job. Somehow, I found time to practice 2.5 hours a day and I still graduated as a valedictorian. Let me tell you, I was pretty busy, but it was certainly possible to get all my work done. The key was this: I didn't waste time! Hope that helps. You can definitely get your GPA up a lot if your work hard NOW. It does matter. Good luck.