With all due respect to the previous posters and their opinions regarding what I see -- perhaps unfairly?-- as "letting the teacher have her way," I would like to politely disagree.
I believe it is, at base, ethically wrong to be marked wrong for a right answer -- and ethically wrong not to challenge it. Oh, absolutely there are worse and better ways to do so, and I love the very polite, deferential ways others have suggested here and think they're outstanding. I absolutely would not, though, let it pass unchallenged for the simple reasons that a) he got it right, and b) others deserve to be taught to spell the word the right way.
Speaking only for myself, as a teacher, I have been wrong more times than I can count, and thankfully, students have occasionally called me on it. I usually begin the year by asking people to see me if there's a discrepancy or I made a mistake -- I'm a human, and strive for perfect accuracy though I do, I can never quite get there. Each time someone's corrected me, it's made me a more accurate, careful, better teacher. No, I don't love criticism any more than the next person, but I'd rather be factually right than "right" because I'm the oldest person in the room.
Just my .02.
