If the students thinks the exam was just a piece of cake, but if he said so, it would be very arrogant, which attitude must be avoided on the part of a student, what should this student say in proper English?
I think the concern about not wishing to appear arrogant is something that does not concern North American/British/Antipodean native speakers as much as it does people from other cultures. Our concern would be rather that we did not wish to appear too confident of success, in case the results showed our confidence to be misplaced.
We might say something along the lines of " It seemed pret