From the perspective of both an avid board gamer and a teacher, using Diplomacy by itself as a teaching tool seems incredibly difficult because it's so slow. It might be simpler than Risk because there's no dice rolling to keep up with, but the operations are still complicated, and what's more, they're slow. The recommended playing time is six hours and I've had games go way longer than that, in part because the negotiations are so easy to simply have bog down. Trying to pull that off in-person seems pretty damned difficult.

Now, that being said, there's more than one way to skin a cat. I would set this up as a play-by-email (of which there are plenty of examples to draw from on the internet: this might be a good place to start: http://www.diplomaticcorp.com/). Break the class up into appropriately sized groups, either on teams per country or (more work but probably more fun) different games and have them simply issue turn orders once a day (or whatever you feel is appropriate) and make them play it out for a week or so. You can sort of sit there and just see how it unfolds and keep the pace moving while also eliminating the need to have them manage the game's mechanics, which is critical: as somebody who teaches a lot of board games to people, getting them to follow the mechanics is where gaming can bog down Even though I can't imagine many teams would finish the game, that would give them a sense of how power politics unfolded, and how they could occasionally explode.