From the American Library Association Video and Copyright fact sheet:

Classroom Use of Videos
Public performances of a video/DVD in the face-to-face classroom is an exception to the public performance right §110 (1) and therefore lawful. The following conditions apply:

• The teaching activities are conducted by a non-profit education institution
• The performance is in connection with face-to-face teaching activities.
• 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.

See full fact sheet here: