I love how she's always playing the long game. Guessing "wrong" so he'll get excited and get yelled at instead of simply losing. Then revealing after that she guessed wrong on purpose.
Also I don't think she did him dirty by teaching him stuff that wasn't going to be covered. He should have known what was going to be covered and she no doubt taught him stuff he would need to know at some point anyway. A few hours of studying the wrong content isn't what tanked his grades. Constantly being distracted in class making stuff like jack in the boxes would contribute far more to his poor performance.