I have had my IQ tested on two occasions. Once as part of a SEN assessment and once when I volunteered to be involved in a research project. On both occasions, my IQ came out as being in the top 0.5% of the population. I have no idea whether my sister's IQ has ever been tested, but I think her intelligence is similar to mine. So I reckon that we were both the kind of kids who would have been considered "grammar school" material.
Except we both failed the 11+. Our parents didn't really believe in selective education and so we didn't do any practice papers. I remember sitting in the test and finding some of the questions utterly bizarre and pointless. The kids who did get in were not necessarily the most intelligent, but they had parents who were dedicated to that goal. Some had been doing weekly practice papers for years. Some had had tutors. There was a local private primary school that seemed to exist primarily for the purpose of getting kids into the selective secondaries.
So DSis and I went to the comp instead. And broadly speaking, it was a comp rather than a secondary modern, because there were plenty of other bright kids there who had "failed" the 11+. And we were fine! The kids were set according to their ability in each subject. There were some kids who worked hard and some who didn't, at all ability levels. I quickly learned that everyone had different strengths, that everyone brought something different to the table.
Going to the comp didn't seem to damage our prospects in any way. Dsis and I both excelled in our exams, graduated with Oxbridge degrees, went on to have successful careers etc. Few of the kids who "passed" the 11+ and went to the grammar did as well as we did academically, and actually, I think they had a less rounded educational experience overall.
My own dd is very bright, straight 9s at GCSE etc. Thankfully we live in an area without grammar schools. There are selective private schools but we didn't think they were worth the investment. So she went to the local comp and she has thrived there, both academically and socially.
There is just no need to separate kids off at 11 years old based on a spurious test that reflects the amount of preparation that has been undertaken rather than actual ability. It achieves nothing, but may leave some kids feeling like failures when their academic careers have barely even begun.