So what do you do? This is the last grade of the semester, and no more exams to study for. A bad grade will make you unhappy for the rest of the evening (you wanted to go to that party, right? You won’t have much fun thinking about that grade). A good grade will make you happy, but so what? Happiness comes with diminishing marginal returns (and for me it’s more like a binary value, happy or not). You have a higher expected utility for tonight if you don’t check your grade. And you’re not any worse off checking the grade tomorrow.

Should you destroy all that expected utility by the truth? (For reference, the truth is a that you got a C-, which is BAD).

I would think that an ideal rationalist's mental state would be dependent on their prior determination of their most likely grade, and on average actually looking at it should not tend to revise that assessment upwards or downwards.

In practice, I think that all but the most optimistic humans would tend to imagine a grade worse than they probably received until shown otherwise, so looking at the grade would tend to revise your happiness state upwards.