Would it be reasonable to suggest that the main argument against Grammar schools is the fact that, (when the Olde King was alive),the division was made at age 11. That ,I think, is pretty young for such a momentous event which, in my opinion , can affect the rest of your life.
If one went to a Secondary Modern but developed sufficiently to sit 'O' Levels it was still possible by going to a College but this set you back 2 years c/f Grammar school pupils.
I don't think social status/affluence comes into it.
As an aside; are people aware that when preparing a 'Personal Statement' when applying to University, to be able to use the expression 'First in Family' (to go to University) is regarded as an advantage.