if there were unexpectedly large discrepancies between what we think people's chromosomes sets should be and what they are in reality we would have noticed by now
And, also, logically, "GCSE Biology" wouldn't then be teaching that sex is determined by X/Y chromosomes, would it?
If something is taught in GCSE anything, it's because it's true, or at least as true as it needs to be for normal situations
A lot of these Dunning-Kruger idiots like to call things "GCSE xxx" implying that it's inaccurate. But the point is that all basic-level stuff in GCSEs is accurate, or there wouldn't be any point teaching it. You'd just leave it to higher level.
(Although, having made that argument, I realise that schools are teaching gender bollocks. But it's not actually a real course - it's being smuggled in from the sides).