The perenial issue here is that what is best for the population as a whole is not always best for the many of the individuals within the population.
Within the broad cohort of school aged children across the whole population, the best educational outcomes will be achieved with a truly comprehensive system where all the cohort are educated together in a school which has the resources to support excellence for those pupils with the capacity to excel, and and to give extra help to those who need it so that no one is left behind.
It is also known that girls tend to have better outcomes if they are in a single-sex environment, but boys do better in a mix-sex environment, so on average everyone does better if schools are mixed, but that population benefit is bought at the cost of some girls not achieving what they might have.
Some very bright and able students are able to thrive in a mixed ability environment but others will only reach their full potential if they are in a selective environment among others of similar ability. It's best for the population as a whole if for the schools to be mixed ability but that good for the whole population is bought at the cost of some very able children not achieving what they might have.
This tension between the good of the many vs the good of the individual can't be resolved. It's natural and obvious that generally politicians who are in charge of the strategic decisions for how education should be managed for the whole population will have different priorities than the individual parents whose children are in the system. When a politician is a parent it is perfectly natural for them to want a different solution for their individual child than the thing that is best for the whole population.
The "problem" here is the fundamental nature of humanity - here we are, a species evolved via survival of the fittest and so with natural tendencies to prioritise our own well-being and that of our offspring over the wellbeing of unrelated individuals, but a species which has evolved sufficient intelligence to discover "enlightened self interest" and devise a society which functions overall for the good of the many rather than the individual. Across numerous topics including gun control, socialised healthcare, justice, education, tax and dozens of other areas, we give up our individual freedoms for the good of the whole but each individual stands to gain or lose a different amount from that trade off, and there are always some who have a bit more to lose that baulk at the cost of prioritising the many over their own best interests. That is never going to change. Even in totalitarian socialist regiemes where individual freedoms are supposed to be totally surrendered to the good of the whole, individuals find ways to advance their own (and their family's) self-interest.