Title 17 U.S. Code § 110(1) provides a specific exemption to copyright limitations on display and performance for the classroom. This allows for the performance of music or film in the classroom if:
Note that all four provision must be met for the exception to apply: face-to-face, in a class, with a pedagogical purpose, and with a lawful copy. Online class showing of a film might be covered under the TEACH Act or Fair Use (see those sections for more). Showing a film outside a class - even educational purposes such as club meeting or lecture series - require a public performance license.
In some cases, works can be purchased with Public Performance Rights (PPR), which license the right to perform the work in public beyond the narrow classroom exception of §110. Very few films are already licensed for public performance, and it is safe to assume that without clear licensing allowance on the DVD case, films purchased directly, rented from a RedBox, or borrowed from a library do NOT include PPR. Licenses that include PPR can often be purchased from vendors such as Swank Motion Pictures (most major motion pictures) or from the distribution company (most documentaries or independent films).