(Original post by Good bloke)
I'll comment as the second person in the thread who has actually paid for his children to attend private school.
The main strength of private schools is that all concerned (parents, teachers and pupils) are united in their desire for a good education, even in non-selective schools. The school's policies are directed towards providing an environment in which children can learn (because the parents want that). The teachers can carry out these policies with a lower level of misguided state interference and get on with the teaching. Discipline is generally very high: the parents don't want their money wasted and the pupils are both self-motivated (generally) and encouraged to work and behave. Bad behaviour, use of drugs etc, would interrupt all this and isn't tolerated.
Some private school myths you may have heard certainly aren't true. Teachers aren't necessarily better paid, teachers aren't necessarily better teachers, pupils aren't spoon-fed (indeed, they are encouraged to think for themselves), facilities aren't necessarily better, and the schools don't prioritise exam results at the expense of other factors - indeed, a big benefit is that the education given is one that would be recognisable to people educated in the mid-twentieth century in that it develops the whole child and isn't just geared to passing exams. Non-selective private schools don't necessarily guarantee good exam success, but they do generally provide a solid education in a good environment for the less able.
An enormous mistake was made in the 1970s when, in an attempt to equalise opportunities for all, and in response to pressure from the left, and instead of improving provision for the less academically able, grammar schools were largely abandoned in favour of comprehensives. Now, instead of 25% of pupils getting a very good and consistent free academic secondary education (which genuinely provided class mobility to the academically able poor), a much smaller proportion of children have such access - and, in the main, it is concentrated among those whose parents can afford to pay for it so it entrenches immobility rather than breaking it down. It is an excellent, if tragic, example of the Law of Unintended Consequences in action.
If we still had grammar schools nationally I'm quite sure a much smaller proportion of children would attend selective private schools, and the same would appy to non-selective ones if the state system managed to get a grip on disciplinary and motivational problems.
The state system should make proper provision for the non-academic, with an engaging programme not geared towards academic exams that are unsuitable for the sorts of careers these people will, in the main, follow. The attempt to make sure everyone can pass academic exams has resulted in A levels without rigour that are no longer fit for purpose, and generations of people emerging into the workforce with numeracy and literacy levels (despite the supposed exam success) that employers complain are inadequate. It has also spawned a generation of people for who deem the sorts of jobs Britain has to offer as inadequate to meet the aspirations they have been mislead into having, so employers are having to look to immigrants to fill the gaps, putting pressure on housing and transport infrastructure and society as a whole.