Your devotion to guitar should help your grades. When you pick up an instrument, or play sports, or get involved in any activity that requires discipline and practice it can only help you in other aspects of life. That being said, being on this side of the fence i.e. not a kid in school any more and someone who has lived a lot of life, you NEED a good education. I know right now it means crap to you but one day you will turn 25, then 35, then wish that you would have done better in school. Good grades are not pointless, you just do not have the understanding yet of what they can do for you. As far as not doing your schoolwork until you get your ax back, come on dude get a life! I assume you are at least a teenager, old enough to not act like a little kid. What next, are you going to hold your breath until you turn blue if you don't get your way? Your parents are concerned for you because they realize how life can kick you in the ass. They only want what is best for you, even though you can not see this right now. One day you WILL understand things better.

You asked for advice so here it is; do your schoolwork, try to understand that it IS the way to a future as a serious musician. Once you get your guitar back, enjoy it and put some of the same energy into your grades as a means to achieving your music goals.