It might be useful to look at the Wikipedia entry for grammar schools before reading this.
The people who enter grammar schools are stardust to the education system. Their parents have the knowledge, time and incentives to train up (or hire someone else to do it for them, in which case they have the money) their child to do well in an exam in the hope that it will give them a better chance in their GCSEs and A-levels. As such when it comes to the actual GCSEs and A-levels, presumably they will do the same – to help their children do well. Why clump all the stardust together? Surely spreading it out is a better option, as it achieves better results for everybody.
But also, why prioritise the stardust? Surely we should focus on helping those most who are least likely to succeed – instead giving them up as a lost cause and instead teaching those who are already succeeding.