I wanted to add that regardless of any other details of the story, calling your son 'Mr. Zero' in front of the class was unprofessional, rude and, in my opinion, bordering on verbal abuse.

Giving him a zero for talking while writing his essay could have a valid explanation. If he had been warned before and the whole class was told to do it quietly, maybe it was warranted, but that 'Mr.Zero' comment was just WRONG.