It was disrespectful, the way it was described. It would also be perfectly normal teenage behavior to say under his breath each time "add", and "subract".
However, it is another thing entirely when a teacher who is wrong, and is confronted in a respectful manner brow beats a child into anger, and then finally admits the child was correct. This happens with very bright children more often than many would like to admit, and sometimes it happens when the child is to young to understand that correcting a mistake a teacher has made is disrespectful. We want our children to be able to speak up when mistakes are made, even if the mistakes are made by an adult. Children need to understand that questioning authority is okay. The attitude of the adult is always right, and it is universally disrespectful to confront or disagree with an adult is not a good message. It puts kids in a position where a "respected" adult such as a teacher, scout leader, priest or coach can do or say anything and it would be disrepectful to disagree.