YABVVVU.
a number of others have covered the key reasons why grammars are bad. @BlouseAndSkirt does as good a job as any.
grammar schools were always a bad thing, that's why they were all but eradicated back in the baby boomers' day, but the case for having them now is far weaker than it was when they were scrapped, due to:
(1) GCSEs - love or loathe them they're by design a very inclusive qualification [at the cost of being too easy for the most academic & too hard for the least], all kids almost without exception do them, so it's not like there's a need for different schools preparing kids for totally different paths; and
(2) The twin growth of inequality [of £ but also of knowledge, contacts, etc] & the private tuition industry, which combine make a mockery of the 11+ as anything other than the bluntest of blunt tools for measuring academic ability.
Non-selective secondaries, with all-subject streaming introduced at the end of year 7/beginning of year 8, is vastly, vastly better at identifying the brightest & best, since:
(a) streaming decisions are made based on the evidence of an entire year during which all kids [average age 11.5-12.5] have been taught the same, not a single one-off exam sat by kids [average age 10.5] regardless of background [including private prep etc] & which is only taken seriously, or indeed taken at all, by kids with the pushiest parents; and
(b) streaming decisions aren't then set in stone - you have 'promotion' & 'relegation' of the strongest & weakest in every set without needing to move schools etc.
look at northern ireland and kent today - are they utopia of high standards, social mobility, etc [a clue for those unfamiliar with either - they're not].