"Can we watch this movie in my class?"
Ah, the joys of 17 USC § 110(1) - the Classroom Use Exemption. (Note, § 110(1) is not an un-nuanced bit of copyright law - it doesn't have anything to say about for-profit classrooms, or online teaching, or a lot of other stuff. But it does cover many of the face-to-face classroom displays and performances that take place at the University of Minnesota.)
Unfortunately, here and at a lot of other institutions, this situation is getting muddier - and the mud is coming from some of the very awesome digital- and streaming-content shifts that I otherwise love!
Watching movies, clips, or other video content in class has usually been accomplished by bringing some sort of video playback device to the classroom - a film projector, VCR, DVD player, etc. But increasingly, institutions are finding the cost of supporting the playback devices kind of prohibitive - not to mention the fact that some content is still only available in obsolete formats like VHS, where playback devices seem like a kind of ridiculous expense to maintain.
So okay, the instructor can play back DVDs via her own laptop!
As long as she actually does that, actually puts a DVD in the drive and plays it back directly, the situation is pretty much the same as using a dedicated DVD player - quite likely allowed under § 110(1) at many non-profit institutions. But...
How many new laptops come with built-in optical drives? And who wants to cart the DVD & computer to the classroom, hook it all up, wait through the interminable anti-piracy warnings on the DVD, wade through the menus to the relevant scene, and possible fast-forward or rewind from there? Ripping a file from a DVD is trivially easy, so many instructors are likely to do that. But that introduces some copyright mud - ripping the DVD might be a fair use, especially if you're copying only a small part to facilitate critical engagement with the content in a non-profit educational context, but there are those who'll disagree about that. Moreover, even if ripping the CD is fair use, it might separately be a violation of the DMCA anti-circumvention provisions.
Similarly, if the instructor wants to play back content from a VHS tape (and there is some really great content that's only available that way) via her laptop, she has to make a digital copy. Although the DMCA is unlikely to be an issue, there's still the question of whether the digitized copy is a fair use.
Further mud comes from the fact that, for "audiovisual works", § 110(1) doesn't apply if the copy shown is "not lawfully made under this title." IF the ripped copy is legal, § 110(1) covers in-class playback - but since there's some uncertainty about the legality of the ripped/digitized copy, there's some uncertainty about the playback of that copy.
Okay, forget about playing stuff that originates on physical media at all - the instructor can use YouTube, Netflix, and Hulu! Or stuff she's downloaded from iTunes!
Um. Did you read the Terms of Service for any of those?
Almost all streaming content services say that their materials are available for personal use only. Whether it is "personal use" for an instructor to play back video for her class is... well, an open question. (Whether YouTube's "browsewrap" terms of service are even enforceable is also an open question, with many commentators leaning towards "no".)
Okay, fine! We'll use content licensed by the University for classroom streaming!
Great. We may have resolved the uncertainty about whether you can use it in the classroom, but we've probably given up ALL THE OTHER USE RIGHTS copyright provides, via the limitations of the institutional license. Also, now that we don't have a local copy of the video content, what are we going to do in two years when the subscription service raises its rates 180%? (These are not inescapable problems, BTW, if institutions NEGOTIATE OUR LICENSES...)
Yeah, so I'm getting less happy to hear this question lately. Evolving technology + copyright law^licenses = messiness.
"Can we watch this movie in my class?"
"Maybe? Possibly? What format is it in? How are you planning to play it back?"