Kay,
If this has all happened the way you state here, and there is no misinterpretation of what the teacher said, then I don't think the "N" on your son's progress report is the problem. Most kids in kindergarten are just learning to write, so an "N" is not the end of the world.
Rather, I see the teacher as the biggest problem. I think this teacher is highly inappropriate for trying to force your son to be right-handed. And to make matters worse, she's trying to do it again for a second year in a row.
Where and when was she educated and trained? This sounds like something my grandmother from the old country (who, if still alive, would be 117) would say---and used to say to me whenever she saw me using my left hand!
It's almost hard to believe that in this day and age a teacher would ever say that she can't help or teach a student to make his letters because she, herself, is right-handed. That's an outlandish statement.
I have many friends who are teachers. One of my best friends is a kindergarten teacher. She's right-handed and has taught plenty of left-handed students these past 18 years. It's going on 1:00 in the morning here, so I'm not going to call and ask her, but I feel entirely confident that she would say this teacher is out of line and needs some retraining, at the very least.
In a very nice, but non-negotiable tone, I would let this teacher know your son is left-handed and that you expect no further attempts to change him in this regard. I would also state my expectation that he will be given proper classroom instruction on making his letters. You should also tell her that you and your husband will also continue to work with him practicing his letters at home.
I am not in favor of getting administration involved when things can be settled appropriately with the teacher, but if this "educator" continues to try and change him and say she can't teach your son because he's a lefty and she is not, then that is a problem that requires administrative action for a proper resolution.
Wishing you a good outcome with this.
J. F.