Don't worry about the school pushing back and generally implying that you are deluded - its a classic technique and something many of us have experienced from time to time.
They do it because its easier to imply that you are the problem, than admit that they are failing your child. If you think about it, if he were to receive a really good education that kept him slightly challenged him most of the time, then he'd need a series of lessons for your son alone.
He's like a little sponge and he'll master whatever he gets taught quickly, demonstrate it in 10 different ways and then sit like an excited puppy ready for the next thing. Even teaching him 1-1 would be exhausting because you'd never be able to rest. You'd spend time working out what to offer next, assembling it into an explanation for him and then a series of tasks for him to do, then he'd have finished the lot almost before you get a chance to start thinking what you can offer next.
Except teachers have 30 children to look after, some of them need things explained many times before they get it, then there is discipline to be taken care of, registers to be marked, art materials to be prepared, games lessons to be devised, forms to be filled, parents to talk with etc., etc. So, they only have a tiny amount of time to teach your child, and since he's easily meeting every government target the teacher is given, she may not feel too inclined to spend time helping him.
Why schools just won't admit that, I don't know. Instead, they prefer to imply the parent is simply wrong about what their child can do, and has the capacity to do. They will declare black is white and if you insist that its black, and provide specialist reports to prove it, they will tell you that they don't recognise the qualifications of the report writer, or that the report is biased because you paid for it. Moreover, they will imply in their manner that you are a pushy parent.
They will discuss you child's progress in levels to show you he has made progress, until you speak back in levels and then they will suddenly switch tack and tell you that you "too hung up on levels".
They will promise vaguely that things will be better soon, or instruct you that they are the professionals and you should leave it to them, or imply you are ruining you child's childhood by hot-housing him (i.e. artificially increasing his ability in specific areas), or they will inform you that you are too focused on the things your child is good at and you shouldn't say another word about them until he is equally good at whatever he has normal ability in.
If the school or an individual teacher chooses to see what is under their noses and actually help your child, then rejoice! However, there will be many years when that doesn't happen and I honestly believe that every bit of effort you put into changing their minds, whether by persuasion or obligation, is just a waste of time effort and emotional energy.
Its great that your son is very able, but the cost is that you'll have to take care of developing his abilities if you want a rate of progress that is anything other than that which the education system is designed to do. (Personally I suspect the state education system is setup to teach children around the 40th centile and everyone else is expected to just fit in!)