Many educators I've met are clueless about mathematics and clueless about giftedness. Part of the evidence for this is the way they stick to faddish math programs and the way that they cling to the idea that if your kid hasn't been through their school's fourth grade math book, he can't
possibly understand long division. More evidence: too many of them disparage approaches that are favored by mathematicians, physicists, and engineers, and fail to even consider that a kindergartner might be capable of doing second grade math. IMO, this is because they simply can't see past their preconceptions, even in the face of evidence. The kid got an A on the assessment exam? Yeah, but didn't get 100%; therefore he failed. And if he'd got 100%, he didn't use
our method. Therefore, he doesn't
really understand addition, and he failed. (Sorry Polarbear, I just can't laugh at this stuff. It's too destructive.)
Bottom line: you can't convince people who aren't interested in listening.

Yes, teachers know way more about how to deal with a heterogeneous classroom than most parents. But this doesn't give them expertise in mathematics --- yet many behave as though it does. I mean, your husband is a mathematician and they question how you and he approach math with your son?
Seriously?