If the work you want to use is not in the public domain and is not covered by the C.O.D. guidelines, you may apply the principle of "fair use" to see if it is permissible to use it.
Fair use provides educators, among others, with the right to use a certain portion of copyrighted acts under certain conditions. Many mistakenly think of fair use as an educational exemption that would allow unlimited use of any amount of a copyrighted work, so long as it is used in an educational setting. That is simply not the case.
Section 107 is a broad and flexible listing of exceptions that may constitute fair use and may be applied to any of the exclusive rights granted to the copyright owners. Other exemptions, listed below, are much more specific. Only exemptions particularly relevant to educational settings are included here. See Title 17 of the U.S. Code for other exemptions
Note that not all four factors need to be met, nor does any one factor have more weight than another: each case is decided upon its own set of conditions.
If you are using fair use as the reason you are making and using a copy keep a record of your reasoning.
The University of Texas, Office of General Counsel has an excellent guide for applying fair use.
If the work you wish to use is not in the public domain, is not covered by the guidelines, and is not "exempted" under fair use, you must get permission before you reproduce, distribute, perform, or prepare derivative works. Proceed to Step Four: Permission.