Quirrell is joking. He doesn't care about the results of the ministry-mandated test, as he already knew what grades his students had earned from him regardless.

Assuming Quirrell did have the final grades already calculated before the test, did Quirrell know ahead of time how the students would score on the test and have the final grades reflect that, or did he use the grades up to that point in total indifference to the test scores?

ETA: Oops, I seem to have completely missed this discussion when I posted this here. My bad.