My daughter is starting at TGS in Sept.
The fact of the matter is that the exams for Tiffin and the Sutton Grammars cover the whole KS2 curriculum but are sat at the start of y5.
Most state primaries will introduce some content beyond y5 curriculum to the kids working at greater depth but are extremely unlikely to be teaching so far ahead that a child at the end of y5 will have already covered the entire y6 curriculum in school. Prep schools might have done but the state primaries in my local area (in the catchment for both Tiffin and Sutton grammars) definitely do not.
Even the brightest child will need to have covered the curriculum to stand a chance in the exam. They aren't going to be sitting there deriving the area of a circle from first principles in an exam- they need to have seen the formula before.
Not all of the exam will be y6 content, but certainly in the exams sat in oct/nov 2022 for both the second stage of Nonsuch and for Tiffin girls a good proportion was. A child unfamiliar with that y6 content wouldn't have passed no matter how brilliant they are.
You can either cover the curriculum with them at home yourself (self tutor) or use a tutor to cover the curriculum. On top of this, there is time pressure and a lot of the other kids sitting the exams will be heavily tutored, so it would be worth doing a bit of work on exam craft and timing - just being familiar with filling out the answer sheets can help with time. For the multiple choice exams, they need to answer each question in under a minute.
I find people are not that open about how much preparation their children have had for these exams. A lot of people who sneer at you for using a tutor turn out to have done hours of prep on their own at home. Other parents go a bit mad, a friend of my daughter's was attending a tutor 6 times a week. She's got in too, and is a clever girl. I don't know that she "needed" that much preparation but the lack of transparency can send people into a kind of madness... It left me wondering if the once a week tuition we were doing was going to be enough or if I was going to send her in terribly unprepared and knock her confidence.
We sent our daughter to group tuition once a week in year 5 with a tutor who was very familiar with these schools. She is an avid reader and consistently worked at "greater depth" throughout primary. The tutor set additional homework that took about 30 mins each night and in the holidays we did extra practice papers to try to speed up her maths which was solid but slow.
She's a bright cookie and hard working but I wouldn't delude myself that she would have got in without the additional work, her state primary wouldn't have got her anywhere near ready despite being in my opinion a very good school- why should they teach a year ahead after all?
I hope she holds her own once she's in- time will tell!