I agree with Kriston and if you can visit and get to know the school and teachers ahead of time and get them to take you seriously - that will probably help you out. I finally, got my son's school in VA to agree to have a gifted education specialist assess my son even though he is only in K. They currently don't even have enough "gifted" 1st Graders to have him join an advanced cluster. There are about 7 identified gifted kids in the entire school. Emotionally, they are worried he will miss his free time in K. For a gifted kid - I see that "free time" - as the time they get to really "think" and be creative in the way they need to be? We are waiting to see if they will agree to "pulling him out for an hour here or there" because he also is having so many behavioral issues - but it's all happening way too slow when you consider what might be at stake intellectually right now.
Of course, if he tests "super gifted" or even "highly gifted" I will probably reconsider the entire situation. The worst for him all along has probably been not bonding with his peers as his language is so in advance of most little kids it ailienates them. Girls, however, tend to be more flexible than boys and adjust to just about every situation (Read - "What Could He Really be Thinking") - however, that often means that their needs get overlooked.
If you already have your private WPPSI - you're ahead of the game. Montessori, though expensive might be a better option. If our Montessori experience had been less disappointing, or if I could have afforded the better Montessori school in town and they didn't have such a ridiculously long waiting list - I still think they offer a better introductory program. (You can view my other posts on Montessori for more on pros and cons). Their math system is so tactile - and in our experience, they encouraged all Pre-K students to stay until at least 1st grade because usually Montessori educated Pre-K's get bored at state school repeating material they already know. Torture for my son right now learning letter of the week for probably the 4th year in a row. Montessori isn't terribly renowned for being great beyond 1st grade though - so in a year or so, you'd be back to square one.
You could also look for an Elementary School using the International Bacc. program. I hear the one in the really "posh neighborhoods" my side of town are trying to introduce that. Don't know to what degree that is more challenging. I've also heard it said State school is still the best place for gifted kids if they are likely to get into the gifted centered schools in your county by 3rd grade - although private schools will mostly take 90th percentile kids only anyway. You could consider private until 3rd grade maybe? Then reassess.
Sorry. I know what you're going through - but I feel bad for not doing more in advance when it comes to getting to know how your local school is likely to react. So - don't wait to get started if you have the luxury of visiting frequently!