We recently did something similar for our now 2nd grader, specifically for math differentiation. We started the discussion during our regular GIEP meeting with the gifted teacher and his regular (at the time 1st grade) teacher. They recommended we meet with our principal and make the request that way, and to let the other parents know we were doing so, but to go in on our own and recommend the other parents do the same.
We had a slightly different situation, though, in that the school had already identified a group of about 8 kids in his grade that were also unusually math-y and were offering a pull-out math enrichment with the school's math specialist. So we could go in and ask for additional math differentiation and indicate we knew that there were other mathy kids that would benefit. We were successful to some degree (extra differentiation, but not direct acceleration) but the process has resulted in the district taking a closer/harder look at what to do with this really unusual cohort and how/if they should change how they math with these types of kids (which are a subset of the identified gifted kids).
I also think we were more successful, rightly or wrongly, because this is our youngest of 3 kids, all of whom are in the gifted support program, but we had never specifically needed to ask for additional anything for the other two kids. We're lucky to live in a district that can provide reasonable accomodation for gifted kids. All 3 of them could easily accelerate in math, but it was clear with our 3rd that he really is a step above and needed more.