I once had a very different, but otherwise similar scenario when I took chemistry in my junior year of high school. After a particular test, the teacher looked at us and said in disbelief:
"I've been teaching for almost 25 years and I've never seen something like this before, ever. Half of you got A's and the other half got F's, except one person who got a C. There were no B's or D's."
None of us could believe it either, but as we shared our scores with our neighbors we discovered he wasn't lying. Teach then jokingly suggested that he should give bonus points to anyone that correctly guessed who got the only C.