Well, when all else fails and I am not interested in the material at all and I think it is completely stupid I resort to a utility argument. Even if I don't find the material interesting or think I will use it, doing well in the class will serve me well in the long term.
For example: In my case the long term is that I want to graduate with honours and get into law school, so I need a good grade in statistics whether or not I agree with the fact that I am required to take it or whether or not I will find what I learn useful in the future. Ergo, I worked hard and pushed through the stats class despite being bored out of my mind and hating it - I got an A.
In your case the long term is a bit different. You haven't said what you want to do with your life, only that it does not have to do with school, but you have your age listed as 13-15. You need to keep your options open in case you change your mind over the next few years. Very few people truly know what they want to do at your age and many people change their minds even at my age and if you change your mind and want to do something that requires post-secondary education, you will wish that you had done the hard work now that would have kept that door open.
Moreover, more and more careers require post-secondary credentials and you might want to factor that into your decision. Out of curiosity, what career are you thinking of?
That's cool and maybe you won't change your mind, but I think it would be wise to have a back up plan in case the entertainment industry doesn't work out. I mean, I certainly hope that it does, but just in case it might be good to have something else to fall back on if it doesn't or if it takes a while to take off.
Also, take the time to explore different fields of study, there are a lot of different things out there and you never know what you will find interesting. Keep pursuing music, but don't limit yourself now. So many of my profs have advised me not to limit myself and that is one of the best pieces of advice I have received.
I wish you luck.
One long-term motivation is financial security and getting what you want. Doing well in school now will help you when it comes to taking the SAT or ACT and when applying for scholarships. Good grades in high school and a good SAT score can qualify you for larger scholarships. That's what happened at my college and I never had to pay tuition because it was fully covered. Even if you're not looking at the long-term goals, just remember that working hard now will make college a whole lot easier when you don't have to worry about money.
I lost motivation in high school. I ended up dropping classes and nearly failing one (the teacher truly only let me pass by her good graces). I was normally a great student but with all the added responsibility in my home life after the death of my father... school got put on the back burner. I still finished though, in the top 10% of my class despite it all.
At least earning your HS diploma is a no brainer. Thereafter, you have more options. But you've got to at least graduate from HS.
I always try to push myself to be the best I can be. Maybe you could make goals for yourself and slowly start working at it, and study longer and harder.
What do you inspire to be?
You'll always appreciate your education later in life. Your education can lead to great things. Ignorance is very costly
Just remember that finishing is important and finishing with good grades can be to your advantage when looking into college. Just shoot for the end result and you'll be fine.