why cant you just concentrate and work hard at school and then come home and play your guitar for as long as you want?when i was at comp (same as high school i think) i did work in huge chunks like 6 hours on a sunday afternoon then had nothin to do for the rest of the week loads of time to practice, when it came to exams it did change a little but not loads i think i just revivised every night from 9 till 11 for 3 or 4 weeks before then once youve finnished think off all the time you will have to practice.
it worked for me 3A*s 5As and 5Bs :cool: was well pleased and my parent bought me a new guitar and amp for getting such good grades
one other good thing one of my friends is really good at music as a subject and because of that his teacher lets him and my band who dont all go to the same school use the music equipment so big plus as it includes loads of amp, a couple of pa systems, minidisc recorder, roland recording deck, ............