Digital media rights for online courses (US)
Per brief linked by @ff524 in the comments (updated location here - PDF), there seems to be a reasonable argument that anything you could do in a face-to-face classroom is reasonable to do in an online learning situation as well, as long as similar barriers to entry apply.

The classroom exemption basically says that what you're doing isn't really a "public" performance per se, it's use in a small-group instructional setting.  Hundreds of thousands of people can't just drop in, like they can for a publicly posted resource on the internet.

As such, it seems that if you use technology to shift that small group around in space (e.g., via online streaming) or in time (e.g., via asynchronous viewing), then it's still instructing a small group.  The legal details might or might not be resolved, but the argument would be strong and there's clearly no moral or ethical issue.

If you just post something publicly, however, then you've removed the barrier to entry, and the number of people who can potentially be using that resource is unbounded.  Maybe there will never be more than just your students, but what if for some strange reason the lecture went viral? While that probably won't happen, there is a moral and ethical (and likely legal) line being crossed when you decide to make something that is otherwise restricted freely available to the entire internet.

In between the two cases, I believe, is a grey area of MOOCs, in which there is a barrier to entry, but the technology is used to greatly scale up the number of students that are being served by a lecture.  Since it sounds like you're planning to stay small, however, that's not something you would need to worry about for now.