As someone mentioned below, some of the sweeping generalisations on here are astounding. Not all grammars are good, not all comps are either.
The idea that at 'all' comprehensive schools the weakest 'flounder' and the brightest don't do as well as they would at a grammar is simply ludicrous. If that were the case then there would be no comprehensive graduates at Oxbridge, Russell Group unis, or in successful high achieving careers. This is clearly not the case.
I am in absolute agreement with Seeker et al, that the major major problem with grammars is that children are written off academically aged 10. It takes no account of the different speeds in which children develop/learn, or indeed what's going on in their lives at the time of the exams.
I would like to bet that there are some comps (in truly comprehensive areas - i.e. no grammars) that perform better than some of the grammars in other areas.
All I'm saying is lets try to be even handed here. It's the biggest truism in the world that some kids will get better exam results from a selective schooling. Some others will flourish in a more mixed environment with a different style of teaching (not worse, just different) and their learning will be enhanced by these experiences (and I'm talking about the bright kids here just so we're clear).
I am very tired of being told that my education was sub-standard because I didn't go to grammar school and that comprehensive school teachers do nothing except 'crowd control'. It's just not true. It's not. I did very very well at school thank you very much and I don't think that by reintroducing a grammar system in my home town, I would have done any better.
Incidentally all I can see that my DH learnt at his grammar school that I didn't was how to do something called 'pole axing' which involved a boy's head and a tree trunk. Not sure I missed much there
... and my exam results are better than his.
Oh and incidentally a grammar education doesn't automatically mean smaller class sizes. A friend of mine has a child at a Kent grammar where their a-level classes are about to go up to 30.