dude... you want to play your guitar right? but your parents won't let you play guitar unless you have better grades right? well then that should be enough motivation right there! good grades=guitar... so just study some more and then you'll get your guitar back. then once you do have your guitar back, just find a balance... if you usually spend 4 hours a day playing guitar, then turn that into 3 1/4 hours and spend the other 45 minutes studying... loosing 45 minutes of guitar playing per day is better then losing all 4 of those hours, right?