Education isn't fair. Whether Grammar / Secondary Modern or Comprehensive.
Some comprehensive schools are great and some are dire.
Great comprehensive schools tend to be in expensive areas. So you can buy your way in by buying an expensive house in the catchment area.
Whether they are great (or get great results) or not may also be because of those middle class parents who tutor, and complain when things aren't right.
But there many dire, frankly scary comprehensive schools, in much poorer areas. For the keen-to-learn DC in those schools it is not fair. They did not choose to live where they do.
Both of my DC went to our local Grammars. A friend had a DS at Grammar and a DD at the Secondary Modern - they got comparable GCSEs and A Levels.
The Secondary Modern is in an affluent area with parents who pay for tutors. The top streams have lots of resources and are pushed. In my friend's case her DD felt super bright because she was top sets all the way. Her confidence grew and she is now headed for a 2.1 in an RG University. If the Grammar School 'lot' were there she wouldn't have been in that position.
Trafford as an authority, with its Grammar system, previously got some of the best GCSE results in the country.
IMO the most unfair aspect is the impact of disruptive children. As a PP said, one of the joys of DC moving on to Grammar school is leaving them behind.
Having your education wasted because of one or two disruptive children is REALLY unfair.
Maybe that's what needs to be tackled.