I have a suggestion for how Diplomacy might be better used in class. I have seen posts about teachers being concerned about maintaining and managing multiple games. This is logistically difficult and also there can be particular games that get unbalanced because particular students lose interest or are less competent.
Instead of having lots of games (which is hard for the teacher to monitor and which leads to some students being lazy and not getting much benefit), have just ONE, but assign roles for each student within the country. This will teach teamwork as well as one of the most important concepts of negotiation for real life, which is that the negotiations that occur on the same side of the table are just as important as those that occur across the table. In real life, more diplomacy is often necessary with members of your own team than with your opponent.
Depending on how many students you have, each country could be assigned a:
Foreign Minister -- Responsible for all official diplomatic communications
Minister of Propaganda -- Responsible for all public press
General -- Responsible for all army movements
Admiral -- Responsible for all fleet movements
Minister of Finance -- Responsible for all decisions relating to builds and destroys
This accommodates a class of 35. If you have more, you could assign the extra students jobs as UN officials, members of the international press, Amnesty International or Doctors Without Boarders, and assign them the job of interfering with the cold, hard realpolitik of the game itself. Their victory condition for the international community students could be that the game ends with at least one neutral center remaining uncaptured and its people free.
All students on a particular team would need to coordinate their efforts, but the student holding the responsible office must make the final decisions for the team, and deliver his/her orders to you (and you will enter them).
This program will also let the teacher tailor assignments to particular student abilities/interests. For example, if you have a student who isn't much into the war aspect of the game, she could be Minister of Propaganda and each turn publish something like why the Galician Rutabaga is endangered, and that is why it is so essential that the region remain a demilitarized zone so that tourists may safely visit it during its blooming season.
Grading could be based not simply on the game outcome, but on the particular written work associated with each team member's assignment. Foreign ministers and propaganda ministers could be graded based on the substance and grammar of their written communications. Military heads and finance ministers could be required to write reports explaining their strategy decisions.
Please let me know if you use any of these suggestions. I am very curious to see how they turn out.