In a few weeks I'll begin teaching a unit that will loosely be structured on a Model UN simulation for 7th and 8th graders.
Each week we have a free period on Fridays that can be used for anything the teaching team comes up with to help with the school community or academics. I was contemplating seeing if I could fit some kind of game session into one of these 50 minute blocks for 20-40 students that would in some way thematically connect to the UN unit? It doesn't have to have heavy academic weight to it, this end-of-the-week period is meant to be lighter and more fun.
The last game I can think of that I played that fit somewhat into this theme was Europa 1945-2030, but that isn't feasible within the time I have. There is also Pandemic as an option, though with the tight time frame it might be hard pull off the level or rule explanation adequately with a large captive audience.
I have no problem getting multiple copies of the same game. The key thing is that I'd need to be able to teach tables of the game in under 10 minutes and have it play for 30-40 minutes.
The first game to come to mind is the old Yaquinto game United Nations. It's long out of print, but it's not terribly expensive in the BGG marketplace or eBay. I don't know how well it would suit. There is a review of the BGG page, but sadly, the rules aren't available. You might see if you can get one copy, look it over, then decide if you want to get others.
There must have been a moment at the beginning, where we could have said no. Somehow we missed it. Well, we'll know better next time.
One option would be Forbidden Island. It's a streamlined version of Pandemic. Much faster to learn and faster to play, so it'd be easier to fit within your time constraints; you could get copies for cheaper than Pandemic; and I'm guessing kids would be more into the components and art than they would for Pandemic.
The only thing is, it would only hit 2 out of the 3 big themes you mentioned (but, that may be enough for the purposes): -Negotiation -Diversity (since each player has a different power).
Also, this is not a purchasable game, but, it's very relevant to what you're doing and you might get some good tips from it. I just heard it a week or so ago on the TED Radio Hour. It's a 4th-grade classroom game called "World Peace Game" that is: a hands-on political simulation that challenges students to work within a global community to address dangerous circumstances with minimal military intervention and achieve prosperity. The TED segment on it is pretty fascinating.
The human race, to which so many of my readers belong, has been playing at children's games from the beginning, and will probably do it till the end, which is a nuisance for the few people who grow up. - GKC
I've tried Werewolf before in a classroom. Unfortunately it's a tough one to pull off when everyone participating is mandated to be there. All it takes is one person cheating and blurting out what happens to ruin the whole activity, and with 40 students, it's pretty easy to have a student act that way.
This is my top choice. It is cooperative, political, and might just be a good fit:
Republic of Rome--Political factions must cooperate as well as compete to ensure the welfare and survival of Rome. This is an excellent game of the cooperative kind. I've played it six times and have enjoyed it. You would have to create simplified rule sheets for 7th graders, I think.
Quo Vadis Ancient Roman Senate, coalition building and busting to gain enough votes to take needed action.
Perry Rhodan--Economics, with players as intergalactic traders. I like this one for its cooperative play element. This would be sure to work wtih 7th graders.
And some others, with various degrees of competitiveness:
Diplomacy—Takes a couple of days but makes the point
Intrigue Renaissance Italy. Very Machiavellian. Could become cutthroat very quickly. But fun.
Cosmic Encounter Space Themed Negotiation--my friends and I enjoyed this when we were in college. It would work for grades 7 and 8
There's also a game called 51st State where factions must cooperative to build a new state post apocalypse after the fall of the US. Haven't played it, but have been intrigued.
I hope you find the perfect game for your students!