Scholastic Incentive System? Why not?
This is as good an example of how an actual economy works as any.
1. Decide on a unit of currency, and its divisions. Monopoly money is as good as anything else, because it shows the effect of "flooding," inflation, and counterfieting.
2. Use your "money" to trade in FICTIONAL goods -- not tangible goods! Trust me, if you go tangible, fights really could break out -- we'll see exactly why in a few minutes.
3. Your school sets up a "salary" for each student, with higher grade point averages getting the greater amount of monopoly money once a week. Out of that, the students pay rent, buy groceries, invest (the financial page works well as an exercise) and so forth. Inside a week, the higher paid students might be in a position to "buy" a rental home and whoever wants to may rent it, etc. Again, use actual market valuations but DO NOT consider using real money.
4. Note that some students are known to cheat. They can bring in Monopoly money from other sets of Monopoly, and there is no way to tell it from the notes actually issued by your "treasury."
At the end of the school year, see who has the most Monopoly money. Then have them account for how they got it, who they got it from, what they invested in, and what the return was on the investment. That is your annual "IRS Audit." Whoever has brought in stolen money, or has "counterfieted" etc, get to spend the last day before summer vacation in "prison."
If that seems like a situation that is rife with fraud, misfeasance, forgery, theft, and other nastiness, then you will have a good idea what it is like to actually balance a real budget.