What is the teacher asking of the student here? I haven't seen anything in the OP's posts about the teacher asking her son to do anything at all in re. Go Ask Alice.
I understand that there are other conflicts between the OP's family and the teacher. But in this case, the teacher sent home a permission slip about a controversial book, the family opted out, the parent mentioned a specific concern about the way the book was being taught, and the teacher defended the book. The OP doesn't agree with the teacher's defense. I don't either. But that's all beside the point.
- The book is controversial, and there are some critics who consider it reasonably authentic.
- As the teacher must motivate her students to read the book, it makes sense for her to defend its educational value.
- The OP's son won't be studying the book, which typically means he will be excluded from all classroom discussions of the book. He won't be participating in discussions where his mom's opposition to presenting the book as non-fiction could create conflict for the entire class.
- The teacher's job is to teach the students. There's nothing in this exchange that indicates that the teacher isn't doing exactly that.
The email the OP sent is clearly antagonistic.
From my perspective, it looks like this issue has become a proxy war for other issues. Some of those other issues sound serious and probably bear further discussion with the teacher and administration. However, sending an email announcing that a student has been made privy to a disagreement between parent and teacher, and that said disagreement is being used to teach the student that the teacher is wrong (teaching critical thinking isn't happening here, because the student isn't reading the material in question and therefore can't evaluate its veracity for himself) escalates the conflict without any possibility of a constructive outcome.
That's really a sticking point for me. This student will not be reading the material that his mom and the teacher are arguing over. He will not be participating in class discussions of that material. So why does this matter? Why is this teacher being informed that the conversation has been used to encourage a student (who already has difficulty with expectations for respectful behavior in her class) to question her expertise? It won't help the teacher with the challenges of teaching the OP's son. It won't help the OP's ds with the challenges he is facing in dealing respectfully with this teacher. The OP's ds can't learn critical thinking skills from something he hasn't read. There is no point to continuing debate on this matter.
I'd be all for questioning the teacher's expertise if the disagreement concerned a matter of settled fact like the value of Pi or the density of water or possibly Darwin's theory of evolution, but this is not a matter of settled fact. While I feel strongly that all the evidence in the debate over Go Ask Alice points to a fictional narrative, it is a matter on which reasonable people disagree. And again, the OP's ds won't be reading the book.