I live somewhere with a 'hyper selective' grammar school with no catchment area. Children apply from up to 25 or 30 miles away, so c. 100 applicants for every place. Those 100 will anyway be the top of the top of their primary schools, so at a rough guess they take the top 0.1% of children.
However, they have had to create special classes and groups for children who cannot keep up with the work once at grammar. Why? Because children have been so intensively coached to a single test - it is quite usual here for children to go to private primaries that teach intensively 'to the test' for 7 years and ALSO have a tutor twice a week for the last 2 years - the test no longer picks up children for their genuine intelligence, it picks up children who have been taught to do nothing else but the test, and struggle when finally at the school.
I refuse to play this game, despite having an exceptionally bright child. I am slowly introducing DS (Year 5) to the format of the test, just so that he knows what to do when faced by it. That is all he will do. If he gets in, then that is because he is genuinely bright enough to cope and thrive and get the enjoyment out of being in that environment. If he doesn't, then the local comprehensive is a good one - one side effect of the 0.1% is that the comprehensive school caters for 99.9% of abilities and so doesn't have the 'cream off' effect that happens in some grammar areas.
My son's teacher, who has 2 sons at grammar school and so knows the system well, is absolutely supportive of, and has totally recommended, my approach. She wishes, as do I, that a test of unknown format was administered 'in school' on an unknown day so that the grammar tests geuinely picked out the children who are most suited to that type of education, rather than the whole industry of private schools and private tutors being built around it.