My school overpaid me in grant funds because of an error made by their bank. I'm afraid I don't have any rights because it's government money, but considering that the school and the bank were the ones in error, do I have any rights? I cashed the check months ago and I have no idea how I'm going to pay it back.
Nope, unless there are some *very* unusual additional facts that you didn't mention. It's not your money, and you have to give it back.