If you then to just be able to play with the filesystem, Amanda's suggestion of a LiveCD is best.
If you need the full install as part of your lesson I would suggest that you either install Ubuntu(any flavor should work here) with the computer offline or uncheck the box that gives you the option of downloading updates. A full install without the most current updates only takes about 15 minutes, it's the downloading that can be the time killer Esp. if the main servers are busy.
If I remember correctly it it the second screen that has the box you will have a list that states
at the bottom of this list will be the box that is preselected that states download updates while installing.
Unchecking that box should speed up you install time considerably.
As I stated above, if you just want you students to be able to play around in a Linux filesystem, I would just use a LiveCD as Amanda suggests, the only drawback would be that any changes made would be wiped out on shutdown, if you need the changes to stay you can use a LIveUsb with persistence. The instructions on how to create a LiveUsb with Windows is here, just move the slider in step 4 to create a persistence file.