If you could turn back the clock to when YOU were 15, what would you practice?
When I was 15, I was just making the switch to piano from electronic organ. I enrolled in classical piano lessons for the first time, but was also just discovering jazz. If it was possible to rewrite one’s own history and change events, I would have switched to the piano earlier, and worked harder at scales and technical studies. I had a lot of “catch up” work to do when I got to university. It also would have been nice to be exposed to the recorded history of jazz at an earlier age. My parents were not musicians, and there was no CD collection of classic jazz recordings in my middle school and high school band rooms. This would have been very beneficial. I didn’t start transcribing jazz solos until I was an undergraduate student, because I didn’t know any better.
To summarize: If I were 15 again, I would focus on mastering the technical aspects of my instrument, study and transcribe recordings, and find myself respected teachers to help steer my path.