How to ask a girl to be more than friends?

[Philip G]

I am a freshmen in HS, and I have a girl that I sit next to in my band class that I REALLY like! We always joke around and make each other laugh. She always pats me on the head (I spike my hair), and I always poke her in the side with my flute or hand. I don't know if she likes me back or not, but I really like her, and I am TERRIFIED of asking her out. I'm considering a note, or having one of my friends do it for me. I don't know exactly what to say or how to say it, and I need major advice.

Also, she says that she's moving to another part of the state, and I'm not sure if it's really going to happen. She has said this about 5 or 6 times in the past, and for the last 3 or 4 she said that it's for sure. Could this be problematic?
We already are really good friends