The Admissions Code ensures that schools cannot not accept applications from anywhere, but also allows them to set lawful admissions policies that prioritise those living in a defined priority area, with the provision that said priority area is also lawfully set. Unlike the other grammar school members of the CSSE, the Colchester grammar schools set no overall priority admissions area. So if your DS sits the CSSE test and you apply to CRGS as your first effective preference and he is ranked highly enough by score, he will get a place there, even from Nottingham, and you will have six months to move your family from Nottingham to Colchester, or at least somewhere
sensibly near. He will be expected to turn up on the first day of term in year 7, even if you haven't managed to do that, but there are rental properties in the town and, if push comes to shove, I'm sure there must be hotels somewhere, although I've never actually needed to go looking for one.
Presumably both you and your DH work from home, or do you work for one of the big multiples which will transfer you from one local branch to another?
The year before last, there was a girl who sat the CSSE test at WHSG, whose parents had got her up at 4am to get there iirc. Perhaps she was from Nottingham? I think the helper to whom she imparted this information was probably just too shell-shocked to ask

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