Showing full online YouTube videos of films in your class may not fall under fair use. We would instead advise linking students to the content that is available on the web for them to watch on their own time.
You may also consider showing clips/scenes to the class. If the clip is of a reasonable amount (such as 3 minutes or less) and provided through a legal avenue (such as the copyright owner's own website/YouTube account), this could fall under fair use assuming all other fair use conditions are met.
Disclaimer: Our copyright answers are intended to guide your own personal evaluation of fair use. They are NOT professional legal advice.
For additional information about fair use, check out the Fair Use Legal Doctrine page from the U.S. Copyright Office.
