Isn't the sheet music already on some site? Like rhapsody or something?
Anyway there are two ways High School bands or any band can get arrangements to play.
-One they buy temporary rights to play the music, but this usually means you have to send all the sheet music back after a period of time and every copy is accounted for.
-Two, they buy complete rights to the certain arrangement, and the school can keep these for as long as they want.
Still, taking the original sheet music(hence why you get photocopies usually) from them, is sort of illegal, but really only if you intent to sell it yourself...I believe. Recreating the music on your own is not illegal unless you intent to record it and sell it as your own. Usually each performance is unique, which is why High School Bands(Marching/Orchestral/or Symphonic) can play these pieces without worrying about legal implications. I would say if they are giving you photocopies of the sheet music, then they probably bought the full rights to the music. If not, they are probably planning on collecting all the copies and sending them back at the end of the school year. This part happened a few times in High School for myself, which was a bummer cause it was pretty cool stuff. Forget what it was though.
Either way, have fun playing.
-Magnum
Edit: Found the site...
http://www.rampancy.net/sheetmusic . It's actually fan made sheet music, so it's nothing official I believe.