I think it's a pretty awesome idea.
The first thing that comes to mind for me here is don't try introducing them to the package manager on their first day playing with Ubuntu. I would just give them a complete tour of the default Ubuntu set up of the GNOME desktop. I'd go through what all the default applications do and where they're at, how to navigate the filesystem and where/how to save their files and how to move files around and such, maybe how to customize the desktop through the System-Preferences menu (Appearance -> Visual Effects -> Extra, wobble the window??? depending on hardware ;-))
And I wouldn't even mention the command-line unless you know the people in your class are interested in it. Maybe offer a special "advanced basics" class where you specify you're going to be going into the command-line so you know your whole audience is on board and engaged.
Pretty close on the heels of just knowing their way around the GNOME desktop, I'd say, should be a tour through "How do I get help?" land - showing them the Help and Support option under System (actually pretty neat, and you can install documentation packages that extend it), The forums, local communities, special interest communities (Ubuntu Students, Ubuntu Women, Edubuntu, etc.), explain mailing lists, maybe IRC, etc.
I might even lead them all through an install process, or invite those who want to bring in their computers to have help on hand while they install it.
Anyhow, you've got an excellent idea. Run with it! Keep us posted!
Simón