14. Currency is used as a means of economic exchange.
Unless you have a mod for Minecraft, currency (i.e. money) is not built in, however, any resource can be used as currency. For example, the most valuable resource, to most players, is diamond. This could be used as the currencies for the communities you have created to work with.
For example, 30 pieces of wood can be worth 1 diamond. There could be students who are bankers that would take the resources and trade out the diamonds. There can be a whole class discussion discussing what the appropriate currency would be and how much certain items could be worth. This would also be a great piggy back off of the trading activity.