Isn't it ironic that results gained in optimal conditions are rejected in favor of results in sub-optimal conditions? I find it INFURIATING that there is such unwillingness to adjust testing conditions in order to maximize a child's performance during an evaluation. The only purpose of that additional evaluation should be to confirm the information that you are providing (I won't even go into why I think seeking that confirmation is insulting--we've all been there

).
I cannot count the number of times I've been told, "oh, she didn't do as well we expected". Really? Could that be because you took my distractible child and tested her with two other students so that independent tasks were being completed while other students were being tested orally? Could it be because the symbolic language in the assessment is not the same as the symbolic language used in class? Oh, you felt it would invalidate the test to explain that 28*3 means the same thing as 28 x 3? Huh.
This whole wait-and-let's-see-how-first-quarter-goes approach shorts children of at least 1/4 of their school year. In many cases, more, because whatever is decided at the end of the quarter (unless you're lucky enough to have access to genuine G/T instruction)then takes another quarter to get "up and running". Or...to not get up and running.
I share your frustration that your teaching expertise is discounted simply because you are wearing your parent hat. It's truly insulting to assume that we would fudge results to bolster our own children. Perhaps more than anyone, a teacher would understand that inflating ability to get a child into a program they aren't actually matched to is a really BAD idea. Do they honestly think that we are eager to put our children in situations that will make them feel incompetent? Really?
It doesn't sound like this school is very open to your concerns, but if you think there's still a chance, perhaps you could go back to them and share in concrete terms why your child is unlikely to show giftedness in the regular first grade classroom. Then propose an alternative: first quarter in a "probationary" accelerated placement and/or gifted program. Then they could evaluate your DS's performance at the end of the quarter to confirm that the placement was appropriate.
If at the end of first quarter your DS isn't able to show his stuff under those conditions (you seem to have questions about the overall public program), then you may want to look at the private option anyway.
Good luck Belle...I'm rooting for you!
Taminy