Had an interesting chat with an acquaintance recently about 6th form entrance to highly superselective grammar.
Despite being ENORMOUSLY selective at 11+, having applicants who travel up to 60 miles a day to attend it, and a 3 year KS4 (ie a narrow curriculum from the end of Y8, leading up to GCSEs), a surprising number don't make it through to the 6th form.
This isn't because the benchmark for getting into 6th form is enormously high - for comparison, over 30 children in the local comprehensive meet or exceed the requirement, which is basically an average of 7 - a low A - across 8 subjects (and for every 8 or 9 you can obviously get a 6 or 5). It's because despite those advantages - very low %SEN, almost zero children living on poverty or disadvantage, zero social problems, very tight range of ability, 3 years to prepare for GCSE while having dropped a wide range of subjects, supposedly 'the best teachers' etc etc, they simply don't make the grade. A handful a year, i'd expect - life disasters, illness, home circumstances etc. But at the rate my acquaintance was mentioning, over 10% of the year group don't meet the benchmark, every year.