I have spent my life in and around grammar schools. I absolutely loved my time at a grammar school. I was on free school meals. It changed my life. But that's just an anecdote. I am no advocate of grammar schools as a system, and there is no way that this policy is designed with the best interests of children in mind.
AND I am sick and tired of this old debate. Why are 'grammars' vs 'comps' the only options? Okay, so there are children who would love an academic environment (they are not necessarily 'more' or 'less' bright than those who wouldn't) - and I don't see the harm in providing them with it. But what about the kids who are brilliantly creative, love art or music or performing arts, and who would enjoy their education ten times more (and I believe that everybody needs some level of academic education no matter what direction they choose to go in) if they went to a school in which this was supported and encouraged? In which they learned their Shakespeare or their history by acting it, or by making costumes for productions? What about sporty kids, or kids who learn best outdoors or hands on? What about the ones, like my DD, who'd learn her maths ten times better if she was working out pulleys and ropes and weights and measurements and angles because she was about to climb a rock face? Why can't we have lots of different schools for lots of different children? (I know the answer: money.) If we're going to 'select' children at all, then we should 'select' every single one of them for their amazing skills, attributes and talents, rather than saying that only one kind of skill/talent matters, only 20% of them are worthy of selection, and that all the rest can be lumped together.
Not sure I believe in that, either. The best thing anybody could do for education, in my view, would be to reduce class sizes to 15. Again: money. But it would make a hell of a lot more difference than grammar schools.