Conventional wisdom is to first check state laws and district policies. You may have already done so, to arrive at the knowledge that it is up to individual schools. Nonetheless, here is a stepwise approach:

Click on How gifted-friendly is your state? in the left margin of the forums, to access the Davidson resource on State information.

Click on your State and view the information. By following the links, you may find current facts you wish to print, such as:

-- Your state gifted association: About Us: Districts: District Info page (contact info). Possibly a district contact from your state gifted association might attend a parent meeting which you organize?

-- Does gifted education legislation exist? Yes Department of Education - Programs for Gifted Children: Program Procedures Manual (15-page pdf). Families may wish to print and read the 15-page pdf prior to meeting.

Is it possible to seek flexible cluster grouping, based on readiness and ability, regardless of age or grade, so that students have appropriate curriculum placement and pacing in each subject, and are taught in the company of their closest intellectual peers? This also simplifies any "differentiation" a teacher might attempt in the classroom as the range of readiness and ability would be narrower.

Flexible cluster grouping is not costly, and is supported by research:
http://www.casenex.com/casenet/pages/virtualLibrary/gridlock/groupmyths.html
http://www.gifted.uconn.edu/nrcgt/reports/rbdm9204/rbdm9204.pdf

Here is a blog post from the website of NWEA, known for MAP assessments. As the article mentions zone of proximal development (ZPD), and "skill-based activity groups" and grouping by RIT score bands are mentioned elsewhere on the NWEA website, when the article supports flexible grouping I take this to mean flexible cluster grouping based on readiness and ability:
4 Reasons Why Flexible Grouping is a Powerful Force for Learning, by Virginia Williams, July 9, 2015.