Congress endorsed, but did not pass into law, an Agreement On Guidelines For Classroom Copying In Not-For-Profit Educational Institutions With Respect To Books And Periodicals. These Guidelines are discussed and outlined in Circular 21: Reproduction of Copyrighted Works by Educators and Librarians from the Copyright Office. The Guidelines provide the minimum standards on educational use of photocopied materials.
The Guidelines do not address all of the principles of fair use and do not take the place of doing a Fair Use analysis. Placing limitations on copying by number of words or by amounts like 10%, do not address whether what is being copied is the “heart” of the work. Also, since they provide the minimum standards, it does not mean that an individual cannot use more than 10% of the work.
That being said many institutions, both K-12 and higher education, have adopted the Copyright Guidelines as a type of safe harbor. The Guidelines are reproduced on the Dowling College website here: http://www.dowling.edu/mydowling/Forms/copyrightpolicy.html
What has clearly emerged from the Copyright Guidelines is a consensus that photocopying of consumable materials, like workbooks, is not considered fair.