Can my department or Media Services make a copy a DVD that I purchased so I can use it in my classroom?
Not without proof of permission from the copyright owner. Making a copy of an entire commercial DVD or music CD for "classroom use" is a copyright infringement. Showing a copied video in the classroom is also a copyright ingringement.
We suggest that you work with your department to acquire a legal copy. Alternately, you can also contact your subject liasion and suggest that the library purchase a copy.