It seems to me that it is fairly standard practice to accelerate gifted math kids one, two, or three levels while still calling them a whatever grader.
Yeah, that bugs me, too. The rich-folks district in our area has enough 6th graders in Algebra I (a 9th grade class in my DD's district) that their middle school planning guide addresses that situation. Our district has twice as many kids per grade, and when I asked about a 7th grader taking Algebra I, I got, "Oh, yeah, that happened once." Not exactly a level playing field in terms of opportunity.
You could choose to accelerate in everything but Social Studies, maybe. (That's got a pretty rigid pacing in my state, and acceleration doesn't really give you more challenging material.) That keeps you in contention with the schools where the highest track is labeled as grade X, but teaching grade X + 2 or X + 3 material. But unless the entire school district is intentionally designed around that system, everything breaks when it gets to your kid.
