It's the one where Lisa cheated on a test and feels terribly guilty, but it won the school a bunch of money so they're not letting her come clean. In this scene, they trick her by having someone dressed as the comptroller commend her for telling the truth, letting the school keep the money, but in reality he's a fake and the real one comes in right after Lisa and The Simpsons leave.
The funny part is I MEANT to make the answer Comptroller Atkins, but I was looking at a bunch of different screenshots both before and after the reveal.