'the whole point of grammar schools was they used to give any academically bright child (from whatever background) the chance to be educated academically.
What's wrong with that as a concept?'
Where do I start? Yes you are taking the most apparently 'academically bright' children (I'll come back to that concept in a minute) and giving them the best and most academic education with other 'academically bright' children and at the same time writing off the other 90% of children as not 'academically bright' and not deserving of an 'academic eduation'. The children not at grammar school will always know they are failures and that they are receiving a 2nd class, not very academic eduation. The kids at secondary moderns were steared towards leaving school at 16 and heading for the pit or to get married or whatever. THey sat different qualifications (CSEs) which, like them, were considered second best. THey never had the opportunity to grow up with and learn from and listen to the ideas of the most 'academically bright' kids at grammar schools. Are you starting to see some problems yet?
Ok, back to the concept of the 'academically bright' child from 'whatever background'. THe problem with this is that the children judged the most 'academically bright' are rarely from 'whatever background'. They are and were almost invariably middle-class. And they have to and had to pass the 11+. Now the 11+ is not some totally objective test of innate intelligence (is there such a thing as innate intelligence or a way to test it? No and no). You had to be able to write essays, do comprehension and do maths etc etc. Now, if you've grown up in a house where learning was valued, where your parents were well-educated, where you were taught to read and enjoy reading from birth, where your family had a rich vocabulary and valued education, where your family had the time and the motivation and the ability to help you learn and coach you for the 11+ and where you were sent to the best primary schools which may well have had one eye on the 11+ you are going to have rather more chance of passing the 11+ than a child who comes from a deprived background who may well have been born with equal intelligence. Can you see that?
Grammar schools have always selected mostly from the middle-classes. A working-class student who got through the 11+ was relatively rare and, in areas, where the 11+ still exists this whole situation has intensified further. If you've had a chance to look at the other thread on the 'Admission Impossible' programme you will know that the pass rate into schools like Wallington Grammar is 18%. That means less than 20 out of 100 kids pass. This means competition is fierce. THe programme showed a child being coached for the test by his father and a paid private tutor for 2 years. The majority of parents who enter their kids for the 11+ will be middle-class, aspirational and very often pushy to a small or great extent. Making the choice for your child to sit the 11+ already says a lot about your ambitions and even if all you do is show your child a sample paper or 2 that's a lot more than many parents would want to or in many cases be able to. And all those parents who say 'I haven't done anything to push my child. If it's meant to be it's meant to be' are not acknowledging that just by growing up with you as (middle-class, fairly educated) parents which most of you will be you are already giving your child a head-start which may or may not be enough to get them through the exam.
Before any one starts, I know there are exceptions. You may well know someoen whose parents worked down t'pit but whose child passed the 11+ against the odds and went to Oxbridge and became a brain surgeon. These parents saved all their money so they could afford the grammar school uniform (which cannot ever be bought at ASDA for £2.99 by the way) etc etc. This grammar-school educated adult will understably always be grateful for the grammar school system. But ask his brother or neighbour who went to the secondary modern how he feels about grammar schools.