Black Wolf, what with the lawsuit-happy world we live in, I can't really advise you on what to do as a 15-year old. I can tell you that when I was 16 and 17, I managed to do things my parents didn't know about. It wasn't preparing for art school, but it could have been were that what I wanted to do.
I know that life is terrible right now and that you think a few years is a long time. But really, you will be an adult soon. Were it me, I would be pretty open about how I don't care to be a JW, no matter how my parents react. I would not give talks in the school nor ring anyone's front doorbell. All they could do would be to drag me to the meetings and to the neighborhood for recruiting. Any punishment seems to be what they would do anyway, like keep me from other kids and the mall and stuff like that. I would probably be well-behaved and silent at meetings and JW stuff so that I wasn't being disrespectful. Then, I could say how they aren't winning me over with their punishments and maybe they would not make things worse. I would keep enjoying music and secretly learn what it would take for me to get into an art school, even if I couldn't really do it for several years (into my 20's).
Flipper mentions any non-JW relatives. Is it possible that at 18, you could go to them to help you on your way to college? If not, then you can still do it on your own. It'll just take a few more years.
Don't be obsessed with "now." It seems just a tiny while ago I was an active JW and now I am almost 10 years out.