^^Agreed^^
Finding an Odyssey would mot likely be a complete pain not worth the one time use your talking about. First find the system hope/make it work, then the cartridges (just jumpers really, the carts could be remade) and finally the screen overlays. If you can find them they will either be beat up or cost way more than anyone should pay.
Here is an idea,
http://www.pong-story.com/odyemu.htm
Emulators don't give you the accuracy or the feel of the real hardware, but your students could at least see the games and play for themselves a bit.
This emulator is old and you will need a dos emulator or a dos/win9x pc to play. If you need help with that than I am your guy. I have things in stock from 486 to Pentuim III.
Honestly I think a $25 pong game would do just fine.