The Fair Use Doctrine is outlined in Title 17, Chapter 1, Sections 107 and 108 of the U.S. Code. It is important to note that just because a copy is made for an educational purpose does not necessarily mean that it falls under Fair Use. There are four factors that must be weighed to determine if a copy may be made without obtaining permission:
1) The purpose and character of the use, including whether such use is of a commercial nature or is for a nonprofit educational purpose;
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 American Library Association (among many others) has prepared a worksheet that can be used to help apply the Fair Use Doctrine.
This is a comprehensive site covering many copyright-related issues including a section about fair use.