Thanks, everyone. And yes, Indigo, I was ready for the advice the next day, so thank you all for that, as well.
I've gone up the food chain, about as high as possible in our school, and am told to expect a response early this week. I've offered the sensible solution of testing out - the plan I thought we had all agreed to, already. I erred in not sending out an email summmarizing the agreed-upon plan. That is excellent advice that I commend to everyone else in similar circumstances. I won't make that mistake again.
The allegation of 'slow work' is entirely bizarre. I do have one of those very slow children, but it's not DD6. She's the fast, accurate, child who has always been described by teachers as a class leader. If she is slow, I expect it's boredom-induced, but I'm inclined to think it's an outright lie designed to make me go away.
If she didn't love the school I'd already be pursuing other options. I may still, depending on how the principal handles the situation. I'm hoping the toxicity is administrator-specific and not system-wide.
