When we started Theatrefolk I took a short course on copyright law. It’s a pretty fascinating field, and like all legal matters, there is a lot of grey area that is constantly being tested in the courts.
Everywhere I go I hear some very bizarre understandings of copyright law. For example, some people think they can perform a play without permission if they don’t charge admission. (Hint: You can’t.) If you change the lyrics to a song then it becomes a new song that belongs to you. (Hint: Nope.) If you change 25% of the dialogue of a play then it becomes a new play that belongs to you. (Hint: Aside from giving the original playwright a massive coronary, this is dead wrong.)
Brad Templeton has assembled the “10 Big Myths about copyright explained” (Eleven, actually.) You’ll be surprised what you’ll find there! I highly recommend giving it a read and sharing with other theatre teachers.