Q. When am I allowed to circumvent or copy media for classes?

The Librarian of Congress is required every three years to designate certain classes of works as exempt from the prohibition against circumvention of technological measures that control access to copyrighted works.  His most recent rulings, 2012, included this exception:

Motion pictures on DVDs or distributed by online services, for purposes of criticism in comment in noncommercial videos, documentary films, nonfiction multimedia ebooks offering film analysis, and certain educational uses by college and university faculty and students and kindergarten through twelfth grade educators.

For more explanation, see the Copyright Office's Section 1201 Exemption Page.