Yikes!! If it helps at all, I probably would have had the same reaction--I am not good in the heat of the moment
But I think you're right--DD(now 10)'s worst year was 3rd grade, when the (private school) teachers insisted that she correct all her mistakes and not make any new ones before moving on...and moving on consisted of adding digits (e.g., going from 2-digit multiplication and division to 3-, to 4-, and so on). That was only part of the problem, but DD absolutely hated it.
I would try to find articles that might support your viewpoint, although I am very sorry I don't know of any off the top of my head, just anecdotal stuff I think from here and Hoagie's Gifted, but the general idea is that gifted kids can often learn new ideas and enjoy doing that rather than getting everything 'perfect' on problems they find boring. Then I might try to schedule a meeting, saying something like 'sorry, I was surprised by what you said at pickup and I would like to discuss it with you.' And then try to persuade. But if that's where the principal is at this point, I don't know how well anything would work regardless of what you said.
Sorry you're having to deal with this!
Best of luck,
Dbat
