The California Student Media Festival has a chart on its web site entitled "Copyright and Fair Use Guidelines for Teachers." Given the organizations behind this Festival -- which include KOCE-TV, Foothills College Center for Innovation, and California School Library Association -- I think the information on the chart has been examined well. I would feel comfortable pointing teachers to this as a guideline, and then to the law for additional information.
The same chart is also available on the North Carolina Conference of English Instructors web site. There they explain that was created by Hall Davidson, executive director of educational services and telecommunications at KOCE-TV in California. A "pretty" version of the chart is available here.
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