Where I live, there is one grammar school for the whole of the city. It takes about 160 pupils per year. The school is 90% full of children whose parents could afford private tuition to pass the exam. The other 10% have undoubtedly been coached at home. About 650 children sit the exam for160 places.
When DD was young, i did consider this school for her. She was in the top 95% in her class dispite being one of the youngest. However, when I realised to give DD a chance I would have to pay for tuition, I decided against it.
My reasons for this was partly financial, it would have crippled us to pay £30+ per week for a tutor. But we could have done it for a short time.
At the same time, I had DS1 who was really struggling to grasp reading and writing in year three. How could I justify paying for tuition for a bright child when my DS could barely write his name. Also DS would need long term support which we definitely couldn't afford.
So DD went to the local state faith school. She is doing GCSEs right now and is below target in most subjects. Her target grades were straight A's. She is likely to get B's and C's.
I honestly believe this is a mix of DD not putting the effort in, (she had a bit of a teenage meltdown during year 10) and teaching methods at the school.
State schools are so focused on league tables, that as long as pupils are on target for a grade C, they are happy. Pupils who could achieve A's but are on target for B's and C's like DD, can be forgotten about because they will not affect the league table IUSWIM.
So when DD got a C in English language at the end of year 10, the school did not want her to resit but pupils who got a D where encouraged to resit. We had to kick up a fuss to get them to allow DD to resit and then they said she wouldn't receive any help. DD did resit and managed to pull her grade up to a B.
I'm sure if she had gone to a grammar or selective school, teachers would have pushed her to achieve her best and not allowed her to sit back because she wasn't affecting the league tables.