The research that no one disagrees with is that the biggest influence on a student's success are parents. That is clearly evidenced on here. Parents who do everything to get the best education possible. It is why that even in schools in special measures, there are often a few pupils getting strings of A*s. To get into a grammar school now is rarely a case of sitting an exam, most have to do some preparation, and parents have to negoiate the system e.g. early registrations etc.
However the success/lack of success for our most able is in part down to Ofsted itself and our exam system and also how Ofsted measures success. Ofsted up until recently have really focussed on data and there has been a big focus on the C boundary, and as potentially one set of poor results can end a head/SLT career, it is no surprise they have focussed on the middle/low achievers.
But the one big flaw in the plan is that we measure progress from KS2 SATS, these are criterion marked e.g. get the question right for that and you get the level. Results (up until this year) have risen and no one seems to go on about grade inflation. However move to GCSEs and the picture changes, if every child who achieved a level 5 made expected progress then 48% (2015 statistics) would be expected to get A/A, (in 2015 16.5% achieved A/A.) The media/government would never allow this to happen, calls of grade inflation etc.
So, we need an exam system/way of measuring progress that doesn't put a ceiling on achievement and allows only a certain percentage to gain the top grades which our current one does. Grammar schools aren't the answer here. A better way of identifying our top pupils, I would argue that success in two subjects that are closely coached at KS2 isn't the best measure. More training for teachers in order to help identify their top pupils and how to challenge them and an inspection regime that looks closely how pupils are challenged (in all subjects as well as the academically able) and encouraging more schools to work together to provide stretch and challenge and opportunities. The training is particularly important, and also then time to implement it. This would benefit all schools and pupils rather than just spending money on opening more grammars for a select few pupils.