Section 107 of the US Copyright Act outlines the four factors to consider when determining whether the use of a copyrighted work in any particular case is fair use:
1. the purpose and character of the use, including if the use is commercial in nature or is for non-profit 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;
4. the effect of the use on the potential market for, or the value of, the copyrighted work.
This website by William Paterson University has resources, interactive tools, and lesson plans for K - 12 schools.