KNOW THE LAW
THE SIMPLE FACTS:
Any work of music that is not in the public domain cannot be arranged without permission of the copyright holder. As of 2016, any music written before 1922 is in the public domain of the United States. Music written after 1922 is protected by the copyright laws of the US. There are some exceptions; however, in order to be safe, it is best to assume that there are no exceptions.
This means that it is illegal for an arranger to write your show if it contains any music that is under copyright protection unless you have obtained permission. I have had first-hand experience with copyright infringement problems in the past, so I speak from the law and (unfortunately) experience. I am also aware that many arrangers will take on work without obtaining this permission and, honestly, the chances of being caught are pretty slim. Many of the copyright owners are lawyers and businesses who would probably not know the difference between a custom arrangement and a published one. Still, in my contracts I include a clause that makes the band director responsible for obtaining copyright permission. At that point, it is up to the director to decide whether or not to take any chances. Before you waste your time, please note that obtaining copyright permission for any John Williams music, or Disney music is next to impossible.
To obtain copyright information you may need to do a little research with the major licensing agencies.
For a list of music in the public domain, go here: PD info