Uggghhh. This is classic prep school behaviour. Very common.
Is it necessarily a bad thing? No. It’s worth being informed now. You could go down the route of questioning the validity of the test- but on the basis that the teacher sees him every day too and so has an understanding of his ability, I would be inclined to listen to her. Yes, from a 92 (depending on what the CAT was addressing), he does fall within the normal range- which also equates to a C at GCSE.
(By the way, most schools don’t use letter grades at GCSE any more, which perhaps says how ‘in touch’ with educational changes and pedagogy the teacher is)
With dyslexia, education IS going to be harder for him. Yes, there are strategies that can be put in place, and allowances which can be made, but depending how dyslexic he is, he is likely to attain lower GCSE grades than his spoken intelligence would suggest.
Keep him where he is if he’s happy- but it’s worth looking at state for secondary. I don’t know the picture nationally, but in my area there is no question that the state schools have far better provision for SEN kids than the expensive public schools (and I work in one of those expensive schools). This is partly because they resource share, have better inspections, are more accountable, and have staff with teaching qualifications rather than PhDs in their teaching subject (only actually more useful at A-level).