Right listen up.
I go to a grammar school and I have done for the past five years. A grammar school setting is not that different to a comprehensive except the behaviour is slightly better and the grades we are expected to get and the level we work at is obviously higher (and the uniforms are uglier). Nonetheless, I personally think that the talk about seperating students being negative is not a good argument.
It is not about differentiating children, it's about providing the facilities for every student to reach their full potential. It's very hard in a comprehensive school classroom for the teacher to cater for a student at the lowest end of the spectrum and a student at the highest end in just one lesson, especially in subjects like English.
If the students who are more intelligent go to a seperate school, it narrows the ability range of a class in a comprehensive, making lessons easier to teach and more effective for students. As well as that, students at grammar schools can have teaching content that can push them further.
I repeat, it's about ensuring every student can reach their full potential - that's why we have schools for students with disabilities (so they can have an environment and work tailored for them to maximise their potential), comprehensive schools and grammar schools.
I don't think I could have reached the level I have reached at a grammar school at a comprehensive; not at any fault of the comprehensive, but because the teacher would have to fit the lesson to the needs of lower-ability students too, therefore not pushing the higher ability students.