That is great news that there are other kids testing at the same levels. I would definitely advocate to keep those kids together.
There is a possibility that you will indeed be pleasantly surprised (I'm primarily an optimist), but it would be nice if the math coordinator would tell you! I can guess at the critical yearness of 3rd - that is when they want your child's scores for the state testing... I would try to get a reason as to why they will not pretest your son. It makes no sense to make him sit through a year of math he already knows.
As for the "wow" factor, if your son's score is just at the 99th percentile, then that's really a great score, but I can see how the GT teacher wouldn't be blown away as it's not super advanced into the upper grades. (But I'm speculating, since I don't know the number, and 99th percentile is still great, don't get me wrong.)
