According to Copyright Law 17 USC (chapter 1) Section 110 (1)
http://www.copyright.gov/title17/92chap1.html#110
110. Limitations on exclusive rights: Exemption of certain performances and displays41
Notwithstanding the provisions of section 106, the following are not infringements of copyright:
(1) performance or display of a work by instructors or pupils in the course of face-to-face teaching activities of a nonprofit educational institution, in a classroom or similar place devoted to instruction, unless, in the case of a motion picture or other audiovisual work, the performance, or the display of individual images, is given by means of a copy that was not lawfully made under this title, and that the person responsible for the performance knew or had reason to believe was not lawfully made;
The American Library Association interprets this to mean the following:
2. Classroom Use of Videotapes
Classroom use of a copyrighted video is permissible only when all of the following conditions are met:
- The performance must be by instructors or by pupils.
- The performance is in connection with face-to-face teaching activities.
- The entire audience is involved in the teaching activity.
- The entire audience is in the same room or same general area.
- The teaching activities are conducted by a non-profit education institution.
- The performance takes place in a classroom or similar place devoted to instruction.
- The person responsible for the performance has no reason to believe that the videotape was unlawfully made.
Copyright for Libraries: Videos and Movies, by the American Library Association.