First. I am so sorry you are having to deal with this.
Secondly I am so sorry that people can be so insensitive to children.
What do you plan on doing about it?
I would suggest if you speak to her, try to be kind. I know I couldn't. But just try to let her know that her comment did hurt his feelings. If you chose to say something to her.
I'm not saying you should or that I would.
But I have a similar issue with my boys and their step father.
He tends to try to bully them into conforming to his wants in their behavior.
He is one of those that will put his face in theirs and yell at them.
I myself have never disciplined them. I talk to them and it works great.
I have a some what tainted past and we, my ex and her husband, don't talk only about my times of visits and small things like that.
I had two roads I could have taken when they told me about how he was acting. One was the old me and I would end up in prison. That's what he deserved.
But I decided to talk to them and give them a way to take his brute force power away without them having to say anything to him.
I needed to empower them so he couldn't intimidate them like he was.
I myself will hit anyone who I think needs it at the time, but I want more for my boys.
So I just told them " He isn't your Dad. He is the guy your mother married."
I also told them to never say that to him. That was for their knowledge not a weapon to beat him with.
This may seem completely different but what you could or may have already do or done is tell him she didn't mean to hurt his feelings. She is actually fond of him as far as you have seen.
People, for the most part, are good at their heart but sometimes they mis-speak and unintentionally say things that hurt peoples feeling.
Then maybe tell him the next time she says something that hurts like that, that he should just look at her and say " I'sa sorry miss teacher.... I'sa doing the best I can with what God gave me.." In a southern accent.
This will do two prominent things. First it gives him something to laugh about with that situation.
Secondly it gives him a tool to express when he feels he has been talked down to that is disarming, funny, insulting but mostly non-confrontational.
It will give him a tool to use to speak up if or when something like it happens again.
I'm sorry that this is so long and if it doesn't apply I also apologize.
I am so livid right now that this happened to you guys.
One thing I wouldn't do is talk to the school about it.
She will just become more hostile towards him feeling that she was wronged rather than see what really happened.
She will get defensive in what she did and tell herself things like " How could he not have seen that was just a joke." It's just what most people do in situations like that.
You are in a horrible place right now.
Some one you love was wronged. His feelings were hurt by someone that is supposed to be aware of what they say all the time so that people like your son don't get hurt.
I really hope that you can decipher this and hopefully find something that will take the edge off your son's hurt feelings.
To be honest I could ramble on with this topic for days.
For one because I am just so F###king pissed that this happened to you guys.
Two, I have a very long history of doing the wrong thing in situations like this and also many years of learning from counselors, probation officers, Judges, ex-cons, priests, family and friends on how to do things the right way NOT my way.
Good luck with this.