I am not sure that you can look at this in the abstract. A certain number of grammar schools exist. There seems little likelihood of a change to the status quo. Abolishing grammar schools would send out all of the wrong signals for Labour and the Conservatives, over 40 years, have set their face against creating new ones.
It might be interesting to think for a moment about that. Margaret Thatcher in a speech to the Party Conference noted that Tory activists would come the Conference full of fire and brimstone about the loss of grammar schools but then, as local councillors, they would abolish the ones in their areas. The reasons were threefold; grammar schools were unpopular with the parents whose children didn't get in (and there really was no alternative to the local secondary modern for the vast majority of pupils who failed the 11+), a grammar school/secondary modern system is more expensive to run and the Sex Discrimination Act was about to mean that equal grammar school opportunities had to be given to girl where previously there were fewer places for girls.
Whilst there is almost certainly a corporate memory of this in the Conservative Party, there must be more to it. One wonders if private opinion polling asking more searching questions than "do you want to bring back grammar schools?" shows that such a policy is still a vote loser?