I know that test scores are part of the determination but it was my understanding that it took in many other considerations as well.
I can't imagine that the school can't at least do a group aptitude test. That disagreement aside, I do like LauraLoo's suggestions. Knowing what the characteristics of other kids who've been suggested for skips by the school are and how frequently they suggest this might give you some interesting insight. Also knowing if their prior assessments of who should skip were "on" might let you know if they are giving you good advise as well.
Like the pp, we were the ones who had to be convinced to skip dd11 and we were approached by the school too.