In writing, shoot a note to the Teacher, and "cc" it to the Principal.
What this Teacher did is just plain mean. Maybe she was trying to teach the class about being thorough in school work...but in this case, I believe, the Teacher went overboard.
I would be FURIOUS if this happened to my child. I would EXPECT the Teacher to answer to Me....with an appropriate attitude... and then I would complain, formally, to her and the Principal...especially if she seems to flip me off so to speak or give me excuses.
Yes, some may say doing this will only set up your son for more "vindictiveness" on the Teacher's part toward your son...but at least you will have a paper trail on it, and know that you attempted to take care of the problem. Retaliation against a child is NOT acceptable...no matter what. Secondly, this will show the Teacher that your Son DOES tell his Parent about things that goes on in class... AND that YOU are aware of her behavior in class, should she do things like this to your son again.
In no way, do I feel this is appropriate for the Teacher to do.
Look, it is affecting your son, emotionally, and his self-confidence. This Teacher seems like a "Bully" herself. NOT acceptable. Is this what you want your son to encounter in class, by the Teacher?
For all you know, maybe she has a history of being like this....but other kids or Parents being too afraid to speak out.
My philosophy is, and which I also teach my children, is to NEVER be "afraid" to speak out when any wrong doing happens... especially anything intentional and "mean."
Frankly, I would be pissed off about this, and go to the school to have it taken care of.
For all you know, maybe the Teacher has done other things to him, that is slowly eroding his morale and enjoyment in school... and most importantly, his self-confidence.
It takes only 1 mean thing to disparage a child and send them on a spiral downward....which can ripple into other behavioral issues for them. Remember that. And then the Parent is left to pick-up the pieces and build up their child again. Is this what you want??? All because of "fear" of a Teacher and not wanting to Correct the Teacher?
If you say nothing... you are not being proactive... nor stopping this "bullying" from the Teacher. Kids need to SEE their Parent doing something about it... and knowing they can TRUST their Parent for taking care of any wrong-doing against them.
This Teacher needs to be corrected. Teacher's are not exempt from "corrections" from Parents. In this case... it needs to be taken care of. You DO need to do something about it. Put it in writing. This is simply NOT acceptable.
All the best,