How big is the class, how much time do you have, and how experienced are the people you'll be teaching?
If you've just got a couple of hours and you're teaching a gaggle of complete novices, stick with a simple extract brew using the sort of equipment you'd get in a starter kit. Ideally, you should actually produce some wort and perhaps share some finished homebrew. If you want to get fancy, you can do a partial grain batch by steeping some crystal malt in the brewing liquor before you bring it up to boiling.