Well, there's always
Isobel's post for a reference, other than that, maybe you could play some goth rock (that's perhaps a little more... classroom friendly), or read some poetry that's considered 'goth' (Poe's The Raven comes to mind), just to give some atmosphere. Now that I think about it since goth-ism so vague (to some... arguably it's very specific to other people... but I don't want to get into that whole debate
sweatdrop ), I think multimedia is the way to go. You could play excerpts from your favourite songs or movies and talk about what makes them goth (thus showing by example). Hm, what else. You could talk about the history of the musical movement, the fashion, and the sub-subcultures within 'goth'. IT sounds like a cool assignment!