"I've accepted a position at another school (don't say where!) which is a huge opportunity for me going forward. Thank you for the opportunity to grow with you. Stay in band, keep working for whomever takes over after me, and I can't wait to see you all at the state marching contest."

Say it at the end of the end of year concert. Make the rest of this year about the kids, not about you. Don't say a BETTER opportunity, don't play up a bigger school or anything else. Just a bland "another position." Every bit of how YOU phrased it in your message shows your move is all about YOU and not about anything else - so keep that ego strongly in check.

Of course, you do need to realize - word WILL get out before you make the announcement, as the position will get posted and people will start interviewing. I'd recommend "I'm not able to discuss that right now." as your consistent response to those questions until your concert is over.