One of the most commonly misunderstood copyright concepts is “fair use”. Section 107 of the US Copyright law allows for certain exceptions to an author's exclusive copyright.
Purpose of the use
Nature of the work
Amount of the work
Impact on the market
These factors must be considered together and weighed equally in order to make a fair use assessment. While educational use is an important consideration, it does not "trump" other factors that need to be considered when using material authored by other people. Amount being copied, presence of digital license, and nature of the work used (how creative is the work?) and how it is being used (distributed/effect on market) must be considered equally when using material. A good explanation of these factors can be found at http://copyright.columbia.edu/copyright/fair-use/what-is-fair-use/.