Have been following this thread with interest... I'm a teacher (State 6th form now) and I fully intend to send my DD private when she reaches year 7. (Can't afford it before, and will spend the next 8 years saving for it!) Why? Because quite simply I feel I owe it to her to give her the very best start I can.
Class sizes do make a difference. I used to teach in a state school with only 25 students per class, then moved to a school with an average of 33. Yet, even I noticed it was that much harder to give all the students the individual time they deserved. In my MA, we looked at research on this, and it does make a difference... especially in the younger years.
But there are also the constraints of the teachers' time to consider... In my subject it is entirely mixed ability classes, and I've taught classes where some students have been doing 'A' levels in their spare time, stuck in the same class with students who have a reading age of 7. I only have so much time, and I have to provide a lesson that meets all of these abilities in one class. What tends to happen is that the lower ability students get extra time spent on preparing their lesson/individual work (otherwise they play up) and you plan the work for the bulk of the class. But what about the really bright stduents? When you teach full time and are only given 3 hours a week to do all of your marking / planning / preparing resources etc something has to give. So what tends to happen is that the bright kids tend to be a lower priority. Teachers know they tend to behave so they get the same work as everyone else and a few extension activities. It fills their time, but does it really stretch them? Sure, these students 'do okay' but are they really getting the education they deserve? My friends in private get loads more time for planning than those in state schools, inevitably they get more time to spend on really stretching individual students, in a way that is virtually impossible given the amount of planning time a teacher in a state school is given.
I teach 'A' levels and I've said this before.. but there are 4 marks that separate out a 'B' grade from a 'D' grade in my subject (per essay that is). Quite simply, the best way to find those marks is for me to go over the essays with students individually and tell them how to do it. Each student obviously has different reasons why their marks are lower than they could be... yet in my contract I am not allocated any time to spend indivually with students working on their exam technique. My friend in a private school is allocated time for this. I choose to give up my lunch hours to help students do this, and as a result I get very good results from my students... but not all teachers do this... yet we all know the difference between getting a handful of Bs at A level and mostly Ds.
Finally, there is the impact of having badly behaved students in class. One or two badly behave students can seriously throw a whole lesson. Whilst teachers can waste hours placating these students (to the detriment of those who do want to work), there are no real means to remove these students. I know of one school where 5 senior managers have to be on duty every lunch and break time to police the school because of one very badly behaved student, which they are not yet allowed to throw out! It is a complete waste of their time, and who suffers? The other children - there are only 24 hours a day and 2 hours taken out to police one student's behaviour means two hours less time to spend on other students.
I could cite hundreds of other examples, but I won't I'll just say there is a reason why so many state school teachers want to send their students private... mainly because we know the truth of the state school system!
(Interestingly, I have only ever taught at so called excellent schools - specialist schools, becaon status, Times good school guide etc etc!!)