There are two main arguments made by some against grammar schools. One is it divides children too early and could leave many in bad schools. There is no reason why the non grammars have to be bad.
In my constituency we have no grammar schools. The comprehensives usually produce results well above the national average, with the best schools producing excellent exam results. The most highly motivated academic pupils go on to Russell Group Universities.
This demonstrates that grammar segregation need not diminish the other schools. Whilst there are no grammars in my area, children from the Wokingham constituency can apply for places at the Reading grammars. The Wokingham comprehensives lose some of the most academically gifted and hard working children to the Reading selective schools. This does not impede them from pursuing their own academic excellence within their schools and producing high quality undergraduates for elite universities.
The second argument against grammars is that low income background children find it too difficult to get in against the competition from middle class children whose parents help them or hire tutors to get them through the entrance procedures. This is too wild a generalisation. It is certainly true that the grammars need to have tests and selection procedures that gives weight to varied levels of preparation, or preferably eliminates as much of the advantage from better preparation as possible.
It is also the case that advantage does not always need money to buy it. A child from a low income home may have parents who provide much time and attention to reading to the child, encouraging the child and engaging the child in the world around them. Some higher income households may have parents too busy to provide the one to one encouragement that can help. Whilst the figures show more higher income household children get in, we should not ignore the non financial support which low income families can supply as well. We need to remind people that any adult with good intentions can help educate a child by sparking their interests or taking time to encourage a love of learning.