I can´t recall something in English exactly equivalent to it. But one option could be "who are we kidding?" it´s somewhat a mild way of lecturing someone, and at the same time it has a "fun" appeal to it (or almost).
Another way, here my educated guess: "No! (Don´t say) I am seeing what I think I am seeing?", something like that, but let´s wait for confirmation on that one. And, finally: Are you sure you are studying? /are you sure this is going to fall on the exam/test?