Copyright on Campus which is a six minute cartoon that does a nice job of explaining many of the nuances of copyright as it applies to educators. While the explanations are given for a collegiate setting they could be applied to K-12 too. The video is embedded below.
Copyright on Campus was produced by the Copyright Clearance Center which is a for-profit organization. That is probably why the video lacks a balanced discussion of section 107 of Title 17 of the Copyright Law of the United States. Rather than making you go look it up, I've pasted the content of section 107 below.
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.