Do a draft of your letter and could you post it here for comments?
Maybe focus on how absolutely vital it is for him not to get hassled and overstimulated. And how a calmer environment will in the end aid his learning, even if the school is less academic for the high functioning children (is that the case?).
also didn't you say it might be only for a couple of years as some children integrate back into mainstream? In which case the ICT issues and working to his full potential would be less important. Would he have access to a computer at Oaklands?