Thanks everyone. I did not contact the teacher right away. The next time I saw the teacher, we chatted. SHE brought up this incident and said something to the effect, "just so you know - we're in charge here - you're not." I wasn't surprised. They school is pretty upset about the partial homeschooling. Mainly they are concerned that others will find out about our arrangement and there will be a mass exodus of families who partially homeschool. (Lots of families know about our arrangement and as far as I know, no one else is interested in doing what we're doing.)
Anyway, a couple of days later I sent an email to the teacher and cc'ed the principal. I just thanked her for her concern about DS. I let her know that we have taught DS that he is ultimately in charge of his learning in the moment. Also, there are subjects that we teach at home and subjects that are taught at school. We follow and support the teacher's rules for a particular class at school and reinforce that with DS. (And this is true.) I asked her politely, to contact us directly if she has any issues that she would like to discuss. Ultimately, DS was kind of disturbed by the teacher's statement and conveyed that to me. I let her know that DS was upset about it and asked that we work together to be sure that this child is surrounded by a team of adults who care for him.
It's not a great situation. I continue to monitor DS to see how he is doing in her class.
