Music in the public domain can be used, distributed, or changed by anyone because the intellectual property rights have expired or been forfeited. There are several places to find public domain sheet music, scores, and recordings on the web - this list is a good starting point. (Always double check that the music you are downloading is truly in the public domain - if you aren't sure, this article can help.)
Great for both school music educators and those who offer private music lessons! Performers may find these links useful, as well.
More resources for teachers! Here is a selection of short articles on some special topics related to copyright and fair use: