Woo, what an interesting post. It connects with issues we're dealing with (DS6 is with his age group although very advanced in maths; with super differentiation, but very much the odd one out - we argued for him to spend some time with much older children, met resistance, and decided not to push it) but you have an extreme case of it there!
My 2psworth would be that a bird in the hand, in the form of a teacher your kids love and a class they're happy in that can continue for another year, is worth several in the bush (in the form of an unproven pullout and uncertain knock on effects). It sounds as though your current school is unnecessarily dogmatic, but you don't have to engage with that: you just need to get the best you can for your kids. It's a pity you can't have both this class and the pullout, but it doesn't sound as though you're likely enough to get that for it to be worth arguing for...
I don't quite understand why you think your decision about the pullout may affect your children's middle school placement? Is it that the children in the pullout may have some automatic skip, whereas if you give that up, yours don't get that? Or is there more to it than that? In a remotely sane system, I would expect that by the school and you exposing your twins to appropriate maths, they'd stay with their peers in the pullout as far as prerequisites and testing went, and you'd be able to make an argument - especially if you have, or can get, in writing that they qualified for it but didn't take it up because of considerations that were mostly to do with their current school. What they'd be missing would be the fun of being with other very mathy children in the pullout. That's important in the long run, but I doubt it's important in the big picture right now.
Are they doing any maths competitions, or can you get them the chance to do them, either through the school or independently? Something like that might fill the gap.
Good luck...
