This is long - apologies. Yes, I am a Bucks resident. There is by no means a uniform pass rate accross the County. Aylesbury Vale has far fewer children reaching the pass mark. The last time I looked it was not at 20%. You cannot look at any school and think that 30% will pass. They may, but it could be 2% or 50%. Years ago I knew one school that reguarly had 75%. However, sitting next to clever children will not guarantee your's passes.
The Aylesbury Vale area grammar schools fill up from out of the County. In Chiltern and South Bucks the pass numbers in the schools is much higher - circa 30% or even higher. I think you have to look at the demographic of the neighbourhood to see what the likely pass numbers could be. How many uiversity educated parents live there and what jobs do they do? How much money do they have for tutoring? Poor children with poorly educated parents are at a serious disadvantage - sadly, in my view.. The statistics are also skewed against the state primaries due to those bright children who are in independent schools. For example, The Beacon school in Chesham Bois reguarly gets 30 into Dr Challoners Grammar - a whole class that are not in the state primary schools. That is only one independent school! The Gateway, Maltmans etc are all focussing on Bucks 11 plus.
Getting the old Level 5 by no means guarantees a grammar school place here. As the new assessments for KS2 are harder, it may be a score well above 100 will be good enough but that is not the only criteria for success at 11 plus! I cannot emphasise enough the need to read widely, have a good vocabulary, be good at maths, work quickly and accurately, read the question accurately and move on if a question is too too tough and takes forever to answer. In other words, exam technique. General knowledge and a quick brain are very useful attiributes. If a child is having to be pushed to get good marks at school, struggles at anything and needs drilling over and over to "get" concepts, then there may be a problem getting the Bucks 11 plus. The secondary moderns do have plenty of high achievers and actually plenty of them do better than many comprehensives, but you have to select the right ones! There is obviously a border zone around 121 where making judgements on what school is appropriate is difficult. Plenty of children do get in on appeal at 119 or 118 but frequently not to the most sought aftger grammars. Below that, it might be better to be top at secondary modern rather than struggle at a grammar. The good secondary moderns are very sought after so live in a catchment for one of them if the grammar does not work out. The Misbourne, John Colet, The Amersham School, Great Marlow and Chalfonts and Waddesdon are all capable of punching above their weight. Many families in Bucks do have a child in a secondary and a child in a grammar. It is quite normal where we live and no-one I knew ever moved to avoid that. I think it is different where the secondaries are not good. We opted for independent for both as we would have had one in each and decided it was not for us but we have the luxury of being able to afford that. Lots of people do private for the one who is not going to the grammar school.
Independent for boys at secondary level is a problem, except in Maidenhead or in the far North at Stowe, but Pipers Corner for girls near High Wycombe is popular. There are more private schools in the South of Bucks than in the central areas.
I do not envy you making a decision. If you are absolutely certain your children are really bright, and you are comparing them against other bright children, then you may find they are ok for the 11 plus. One of the biggest mistakes is to have children in a so-so primary school, find they are on the top table and believe that is 11 plus guaranteed, but in another school, that table would be mid division. It is very easy to under estimate how good children are in other schools that you simply do not know about!