1. Take any reasonable workstation you have laying about ( with at least say a gig of ram in it and load up SLES 10sp1 and plug it into your LAN, give it a static IP address.
2. Install Directory Services on it, and join your current tree or have a stand alone tree, your choice ( full install of OES2 if you like )
3. Install GW for Linux on the box and create a new GW system ( more then one can exist in a tree ), heck DL version 8 beta 5 for that matter (all older clients will be able to connect ) , create ONLY a PO and a DOMAIN.
4. Create your training users as obvious fake names as ** EXTERNAL USERS ONLY ** within the alternate GW system. Yes they will be in your tree but ONLY as external users so no impact on actual user accounts.
You now have a "Teaching System" and your cost has been the box and a few hours of your time. Best of all it is completely self contained ( if you don't install it into your current tree) and will not affect your current GW system, and you can flush all content when you are done and are then ready for the next class.
Best of all you can just power down the box when it is not in use.