I think the teacher's response would have been damaging to BOTH my children, my Aspie AND my NT. My Aspie because writing is a HUGE chore for him, very strenuous, and my daughter because she is a perfectionist, and really beats up on herself for every imperfection.
The better response would have been, "wow, what a lot of words you found, and so fast! Maybe next time you can write them a little more slowly and space them better so they are easier to read."
As for how to address the teacher about, I am not sure. I have not encountered this sort of problem. Perhaps just letting her know that you were really surprised at what she said and how she said it, and opening a discussion that way. It is always possible that she was just having a bad day.
_________________
Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).