It's like this; when I was 17 my parents told me a million times that education was the most important thing. I refused to listen because I never heard any compelling reasons to believe it. It's only now, as an adult that I see why school was the most important thing. Not only for future job prospects, but for social reasons as well. As you will eventually see, the older you get the less anybody in the world gives a crap about you. Right now you are self centered. One day you will not have your parents to tell you what to do and you will have to make your own way in life. If you had poor grades, or dropped out of school you will get kicked to the curb and nobody will even pause for a second to care. Life moves on with or without you so it's up to you to decide if you are going to participate or just be a rebellious loser. The whole "I'm an angry teenager and I do not need to listen to anybody" thing has been done to death. It's very played out already. Be a leader, try for a second to understand that advice that mom and dad give you is a gift of wisdom that you cannot see yet. Have a little faith that they are not trying to steer you in the wrong direction and that they only truly want what is best for their kids.
Be smart enough to figure a way to get what you want, your guitar, but still do the right thing by them. Kids think they have all of the answers anyway so challenge yourself to figure out how to get back the guitar, and get decent grades. Even if you are too blind right now to understand the long term effects of good grades, at least just do it to get back the guitar and see how the whole thing plays out.