How did the grading work? My first guess was that Quirrell was telling the truth, but this seems unlikely, and would mean that the EE+ he gave Harry couldn't mean anything. My second guess was that Quirrell came up with the grades before-hand, but all he knows is how well they could actually defend themselves, and real-world ability is not a good indicator of how well you'd do on a written test. My third guess is that a Time-Turner is involved, and he'll have Harry deliver the tests to him in the past.