Hmmm... this is a tough one to answer. Obviously, it's pretty unusual to have little interest in sex at 15 - this is a time in most guys' lives when Testosterone levels are quite high. The only things I can think of to ask are...
1) Has anything significantly changed in your life recently (say within the last few months up to maybe a year ago)? Are you under stress at school? Did you have a bad breakup w/ a GF? Did you have any type of serious illness that caused you to miss quite a bit of school, or required hospitalization?
2) Have you had any problems with depression, feeling sad or tired all the time?
Things like these could have an impact on your sex drive & libido. If none of these seem to apply, I'd wonder whether there might be some underlying condition (pituitary, thyroid, etc.) that could be interfering with your body's normal Testosterone production? In that case, you'd want to see the Dr & I'm sure he/she would order some blood tests to see if anything appears out of whack.
I doubt anyone other than a medical professional can answer this question, and even then, I think they'd want to do a physical exam & some lab work before coming up with a solid diagnosis. My advice would be to make an appointment w/ your regular Dr. Good luck...