Copyright law places a high value on educational uses. The Classroom Use Exemption (17 U.S.C. §110(1)) only applies in very limited situations, but where it does apply, it gives some pretty clear rights.
To qualify for this exemption, you must: be in a classroom ("or similar place devoted to instruction"). Be there in person, engaged in face-to-face teaching activities. Be at a nonprofit educational institution. Sounds a little restrictive?
If (and only if!) you meet these conditions, the exemption gives both instructors and students broad rights to perform or display any works. That means instructors can play movies and music for their students, at any length (though not from illegitimate copies!). Instructors can show students images, or original artworks. Students can perform arias, read poems, and act out scenes. And students and instructors can do all these things without seeking permission, without giving anyone payment, and without having to deal with the complications of fair use.
Source: University of Minnesota Libraries. Creative Commons Attribution-NonCommercial License.
Copyright in online learning and teaching environments is governed by the Technology, Education, and Copyright Harmonization Act (TEACH Act), which was passed in 2002. Because of the different way of sharing materials in an online class, the TEACH Act applies to the transmission of materials online. There are many requirements to meet for an action to be protected by the TEACH Act, but other exemptions, like Fair Use, might apply better or more easily.
Many libraries have published TEACH Act Toolkits to guide instructors in using copyrighted materials ethically, responsibly, and legally. The original TEACH Act Toolkit, from the Louisiana State Libraries, provides TEACH background and explanations, checklists, guides, vocabulary, and commonly asked questions. Touro College librarians are also available to assist in assessing the applicability of the TEACH Act to your situation.