Hi
I studied Music on my own for my Leaving Cert many many years ago. I also did piano and violin. I taught Music for the last Intercert - so that's a very long time ago. I went back to University after 10 years teaching Music to do a Science degree and have since been teaching Maths, Geography, and Biology part time.
My advice to you is keep all your subjects for this year. By the end of the summer exams you will know which subjects you WANT to sit down to study. Like music - you either feel like doing your scales before your pieces or the other way around - what feels good after you've given them all a fair chance is what you should go with.
Don't think about 'to drop', think about what you love to learn, what interests you and tick them off your list giving each a priority level. If something has to go, let it be the one you like least that has the lowest priority level. But don't let it go until after the summer exams when you will be in a position to really judge the subject.
Your musical training will have given you good self-discipline and the ability to work on your own, so play to your strengths. 30 minutes study per subject is more than adequate (I used only do 20 per subject because music practice took up so much time). So if you are doing 8 subjects and give them AT MOST 30 minutes study per day - you only need 4 hours study. Alternatively, work out how long you will spend on piano(an hour at grade 7, less if you are particularly talented with good technique) and violin (30-ish mins at grade V). Now how much time have you left to study? Leave time for your meals and at least an hour for relaxation and recreation OTHER than music - something physical - a walk or dancing preferably with friends who are also good at time management. Divide the time you have available evenly between all 8 subjects. Even if it means you don't get all your homework done, give the same amount of time to each subject.
(If the teacher challenges you, you can show him/her this. My French teacher used to give a lot of homework. I was very good at French and did it along with Music in UCD, but at school the teacher wanted to know why I hadn't all my French homework done. Like you - my parents wanted me to do well at music. So I said to her "it's like this, I have 20 minutes to give each subject and the work you set took more than 20 minutes so I couldn't fit it in". That teacher was the most feared and the "bitchy teacher" and everyone was scared of her, but when I put it to her logically, out-lining my priorities and my time management she accepted it. Everyone else was too scared of her - they used to spend so long at French that their other subjects suffered.)
My friends would complain that I never did any study and I always got A's. Let me tell you now, I am not naturally a genius - but I always liked learning and I kept to my time management strategy, including a 5 minute break in between subjects and it works. It is a fact that the mind can only concentrate at the most 40-45 minutes at one go. So work with your own body as you do with music. Relax, focus for 20 minutes (or 25 or 30 depending on bed time), break. Next subject. There is always enough time - if you need more advice on how I did it, let me know. Remember that sleep is essential and use tricks like turning off the music, write out what you want to learn off by heart and go to sleep - your brain will keep working on it while you sleep.
On a side note - it is scientific fact that it is ok to have Mozart on but not Stravinksy (due to the relationship between regular four-four time and brain-waves) when studying but that's another story.
I am still studying - I'm a perpetual student. Just today I found another course I want to do on Greek and Roman Mythology with the Open University. My point is that I had to re-learn HOW to study EFFECTIVELY, because I had forgotten how important time management was. Self-discipline and a clock will bring you where you want to go. I wish you every success on your journey. Don't forget that you have a great resource here on All-Honours - we're all here to help.
Best wishes
Carima