Hi there,
We have a lot in common in terms of our school districts, and we have just (we think, I won't really believe it until he is in the classroom) successfully advocated for our son to get subject accelerated in math 2 years, so here are three suggestions.
The first thing I would suggest is to find out what the district policies are on levels, grouping, etc. especially as they hit middle school - for instance, in our district 5th graders can take a test that allows them to skip 6th grade math and go directly to honors level 7th grade math. That sort of thing, even if it doesn't directly relate to your child yet. If you are fairly certain you need to stick with your public schools (we felt that way), start researching all state and district policies re: gifted education, levels, AP standards, etc. Try to connect with your state gifted organization as well and see if they know anyone locally who has successfully advocated for their child in your district. Assuming your child tests "gifted" (which it certainly sounds like), you will need to know the policies, etc.
Second, I would strongly suggest that if you are going to do testing, you bite the bullet and do both an achievement test like Woodcock Johnson and an IQ test. Our son's IQ test certainly was a useful piece of the puzzle, but most schools don't care that much about them (especially in states that have limited G/T programs). Achievement tests, however, will often make them sit up and take notice. I regret not doing an achievement test first (or doing both) as I think it made advocating for our son more difficult. The schools were not familiar with SCAT, so that didn't count for much with them. While ultimately the IQ test was sufficiently high to convince them (after months of pushing) to test his math skills on an in-district assessment, I think WJ would have helped move things along faster.
Third, don't give up. We were initially told that the district doesn't do subject or grade acceleration. As I talked to people and a very supportive teacher from an earlier grade, I found out that there had been very occasional (maybe 1 time every 3/4 years) cases of both. After reading the district policies closely, it was apparent there was no official policy re: acceleration, which gave us hope - even if it meant taking our efforts all the way to the Super. It didn't take that, but it did take months of calls/emails/visits to the principal, and then ultimately going directly to the districts K-12 math curriculum trainer, at which point they agreed to test DS in district. From that point, things did start to happen. My only regret is that I didn't push harder/faster. My new rule of thumb is that someone gets 3 days to answer an email or phone call, 1 week to take a promised action and if I don't hear from them I will contact them with a "what is the status" type email. If I am not satisfied with the results after 3 interactions, I will move up the chain of command. I spent 3 months dealing with the teacher, and another 4 months dealing with the principal - never again.
Good luck. I think it is important to try to remember that most schools really do have the children's best interests at heart. At least in our district, 50% of the parents think their child is in the top 5%, so we have learned that our district is very skeptical of claims re: "giftedness." Indeed, we were told that one of the reasons they had concerns about IQ tests is because they believed that some number of psychologists "fudged" results. Which, given our part of the country and the emphasis on getting ahead, doesn't completely surprise me.
