So the OP blurts out an answer to one of the test questions (yes, it was a bonus question, but it could have easily been a question written on the test itself) and you think he should get off scot free? He broke a rule. The punishment for breaking that rule is you get a zero on the test. Simple. If you break a rule, you have to pay the price for breaking it. It's pretty cut and dry here - but that's just my opinion (and probably the opinion of the teacher and the principal).
Sieg Heil!
He didn't cheat on anything. He accidentally blurted out an answer without thinking on an extra question.
Did he cheat on the test itself? No.
Did he have a paper hidden giving him answers? No.
Was he talking with other students trying to get answers? No.
Was he peeking at another person's paper? No.
Was he using an electronic device to get answers? No.
He didn't cheat on a test.
The black and white notion of "you broke a rule" is pure idiocy. Black and white thinking is the worst kind there is.
He blurted out an extra credit answer by accident. He didn't cheat on the test he'd already finished by himself. He didn't give answers to any other regular questions. His tongue slipped for one second.
You. The Teacher. The Principal. Everyone agreeing with the teacher is flat out
WRONG.
Let's try not to be rule Nazis here and instead take each case as it comes. Clearly in this case, he made a little mistake that could have easily been corrected by changing the question at hand. Instead the teacher decided to go ape [censored] and be a total [censored] which was completely uncalled for.