Hmm... tough question. I disagree with Warb's assessment. For me, Casablanca is a film that requires prior understanding of filmmaking from the time. It's too theatre in its acting and set-up for a modern group of high-school kids to enjoy unless they already have a love of the medium, the 40s, etc. (could be one or the other, could be all).

With only two hours, I'd consider going decade-by-decade with clips from famous movies/directors/etc. Or, perhaps, by genre. I'm afraid my knowledge of film extends mostly to cliché factoids or landmark achievements, otherwise I'd have a list of suggestions in that regard.

I hope you get the chance to do this! Could be really fun.

edit: I will say I like your idea of comparing editing techniques. It's definitely attention-grabbing and may be accessible beyond just your future film buffs in the class.