I am not sure what admiddions criteria you are referring to, Tulip81, but if it is Waddesdon, you need to remember that the catchment area extends very little beyond Waddesdon, to just a handful of neighbouring villages, and the other chldren are chosen by affiliation to C of E. They may come from as far as 10-15 miles away - and definitely South of Aylesbury! There are children that have to travel through Waddesdon to get to their schools in Aylesbury but cannot get into Waddesdon.
You can check the consistently good secondary schools by looking at the number of Ofsted reports that have declared them to be grade 4 or now, grade 3. Some have a long history of this, e.g. Buckingham School, Burnham, Cressex, Quarrendon (now Aylesbury Vale Academy). Other such as Misbourne and John Colet have blips but, especially Misbourne, are consistently pretty good. Chalfonts and Waddesdon are consistently great. Great Missenden has a fantastic primary too as does Little Kingshill and Little Chalfont.
Believe me, the grammar schools only take pupils who will do well in a grammar school but they do not want unfilled places because, obviously, that is less money for them in a time of real budget constraints. Bucks is a very low spending authority. Therefore any child who is a few points of the pass mark can often be offered a place on appeal, but at 25% plus of places being taken by out of County children, there is no real evidence to support that local children could have obtained these places. They just did not get close enough to, or above, the pass mark. If you look at Aylesbury Vale, there are 4 grammar schools and 6 secondary schools, plus a small free school. I will look at the admission numbers, but last year the initial pass rate in Aylesbury Vale was 16%. If the grammars did not take out of County children, they would be making drastic changes to the schools in terms of teaching staff, subjects offered etc. They NEED these children to keep the numbers up and the results comensurate with grammar school expectations. The 11 plus is now an exam chosen by the grammar schools, not the LA. It is what they want. They also want the brightest children and they do not care where they come from.