Lie and say you don't know (but hope that somebody else tells Professor Flitwick the truth). Tell Professor Flitwick that he ought to ask your classmate (and resolve to tell your classmate that if he doesn't tell the truth, you will). Tell Professor Flitwick the truth. If your classmate is prepared to win by cheating, he deserves to be found out. Also, as you are both in the same house, any points he loses will be regained by you, for coming first in his place. You would not wait to be asked to tell Professor Flitwick the truth. If you knew that somebody was using a forbidden quill, you would tell the teacher before the exam started.