Disclaimer: I am not a lawyer, but I did study music copyright law in college 20 years ago. If you want to verify my assertions, consult an entertainment lawyer.
In the USA, you cannot copyright your own arrangement of a song that is already copyrighted by someone else. It is unlikely that you could obtain approval to make your own arrangement from the publishing company that represents the original composer. Therefore you can't sell or redistribute your arrangement in the absence of such an approval agreement. However there is nothing wrong with your making your own arrangement for the use of one performance by one band without a formal arrangement with the publishing company.
You cannot make and release recordings of your arrangement unless you pay a mechanical license fee to the publishing company that represents the original composer, paid through the Harry Fox Agency. Contact the Harry Fox Agency if you want to pursue this.
Your high school band can perform your arrangement of the song according to the blanket licensing or contractural arrangement that the school has with the performing rights organizations in the USA (ASCAP, BMI, SESAC). If such an arrangement is in place, the original composer gets royalties. If your school system happens to be exempt from payment by contract with these agencies, then there is no problem. But I do not know what kind of licensing arrangements schools and school systems in the USA have with the USA performing rights societies these days.
If you really want to cover your bases, you can identify the publishing company that publishes the original composition and contact them and ask them for permission for what you are doing. But if you are just doing a one-off arrangement for one high school band's performance, they may not even bother talking to you about it.
Again, I'm not qualified to give any legal advice. These are just my ideas based on what I know about performing rights and copyrights.
