The assistant made a snarky comment about how it doesn't matter what they teach him, because he doesn't pay attention in class anyway.
My oldest is now in college, my younger in 6th grade. I have heard different versions of this for as long as my kids have been in school. And from my own experience - especially from what I learned when I did nothing - I completely underline the advice you've already been given. What I would do is email her with a cc to the principal and to the special ed chair, or whomever it is who follows your son if he has someone, restating what happened so that it is documented and let her know how concerned you are about what this type of attitude might be doing to your son's willingness to take risks with learning in her classroom. I would also likely ask the question about whether this would ever be considered appropriate for a child with disabilities.
One sarcastic teacher sent my older one on a spiral that took almost a year to repair. He decided to teach her just how stupid he was by getting zeros on everything he did - by making sure every answer he wrote was wrong, not by turning in blank sheets. She never once picked up on how smart a kid has to be to get every answer wrong (he has to know which answer is right and not pick it). She said he was a slacker, should not be in gifted, etc. I didn't take her to task, and I am so sorry I didn't. I not only kept her from being made accountable, it also showed my son that I wouldn't advocate for him for the sake of peace.
This year, I've had to challenge two of my younger son's teachers. And one continues her passive-aggressive snarks, but each and every time, I make sure my son knows I'm taking it seriously. He also knows it means he has to be accountable as well.
Go put on your momma bear ... I'm definitely in your corner.
