Broadcast a copyright-free work is legal.
It can be works that have fallen into the public domain, such as Buster Keaton’s films, or modern works placed in copyleft, such as documentaries, movies by Nina Paley or animated movies from the Blender Institute.
A resource website with copyright-free films (and other resources) : https://libre.seedtohumus.org
You can also take the opportunity to talk about free culture and copyright issues.
Poster dedicated to the presentation of Creative Commons licenses
Website dedicated to an exhibition on free software and free culture