For girls the schools in this neck of the woods, ie south Bucks, are Berkhamsted, RMS (both ind) and Dr Challoners High (grammar). DD1 going to the latter next year. We also considered St Helens in Northwood (probably the most academic school in the wider area, although a bit of a trek) but on second visit found the atmosphere a bit assertive - and frankly one of the things that we were considering when looking at private schools was the smaller class sizes and pastoral care so that seemed to mitigate against it. (Having said that, others will love it, it's so difficult to decide from the 'outside'. RMS seems very hot on pastoral care and attention and Berkhamsted also emphasises that they will get a tutor to oversee them and smaller class sizes).
For boys, here there is Dr Challoners Grammar and in Wycombe RGS. Rather fewer private options - certainly the presence of grammar schools makes the area less attractive for the indies. There is Berkhamsted Boys, and Merchant Taylors in Northwood - the latter very good but again a bit of a trek. For boys the local prep here is the Beacon. There is also (see recent thread) a very good state primary here and in Little Chalfont - sadly we are not in the (very small) catchments.
Our local 'upper school' (ie sec mod) not so great but Chalfont Community College supposed to be very good.
One big thing about this area is a certain 11 plus obsession - almost inevitable given the system. The first thing a good friend said when we moved in was 'Do not assume that your child will pass the 11 plus' - a very good warning. The pass rate is higher than Kent at 25-30% but the area is increasingly full of 'motivated' parents and the coaching system is rife. It tends to hang over your head and can produce a lot of pressured children and upset. A factor to take into account when looking at the 'great grammar schools' in any county.
Anyway I've said enough - lots of luck with your choice OP and whichever school they go to, I hope they learn a lot (as well as passing exams).