I disagree, a comprehensive education MUST include setting or streaming. That way, all children have the same opportunities but the actual teaching is targeted at their needs and abilities.
So for example, my children at secondary school are in a mixed-ability tutor group. They stay with this tutor group for 5 years and have some of their lessons as a tutor group.
However, for other subjects (maths, English, science, languages, humanities), they are set in ability groups on each side of the year (year group is split into left and right sides).
This means that tutor groups have a broad mix of ability, talents, gender, social background etc. It means that someone who is very good at say maths but struggling in English can be in a high set for one and perhaps a lower set for the other, thus being challenged and supported at the right level.
Sets are reviewed for each subject each term, and there is a certain amount of fluidity, so being in a set is not being set for life.
Some children also mature later than others academically, and setting allows them to move up through the groups if it is appropriate. That's why selection at age 11 does not work for all. Some European education systems are particularly bad at this, looking at children aged 10-11 and deciding what sort of career or job path they will be suited for.
My own sister in law struggled with science in primary school and the early years of secondary school, getting lower than average SATs results. However, she then seemed to really take to it and develop a flair for science. She moved up through the sets, did very well at GCSE and A level and is now finishing a science degree.