Thank you for taking some time and thought with this issue. I will be cautious in all instances. When I ere it will be not be with any malice, just my normal wondering and wandering.
Enjoy the journey.
The fair use guidelines for education allow you to use 10% of a copyrighted work, or 3 minutes, whichever is less and the clip cannot be altered in any way. Although I am not a lawyer, I have been adjunct for several colleges and have never encountered a problem with this guideline as long as it is expressly for academic uses.
One of the things Renee points out in her work is that no "rule of thumb" like this is really helpful, one must look at the context and purpose.