I went to s selective private school followed by the local state 6th form.
I'd say that the mix of great and terrible teachers was about the same in both. The results of course were very different. There are so many reasons for that.
The private school was selective and only allwed you to do an A-level in a subject that you got a B or above in at GCSE. You couldn't do a science A-level if you had done dual award science GCSE you had to have done separates. The state 6th form on the other hand allowed anyone to do anything, there were people in my physics class who had failed maths GCSE but were permitted to take physics A-level. Obviously things like that will greatly affect the result.
The teachers had to teach at a level that included the lowest ability in the class, thus not pushing the higher ability students. I
n addition to this the private school was stricter, homework had to be done to a hight standard, they marked far more harshly than the actual exams and it was drilled into us that A was good, B was acceptable but C was fail. People worked hard and were pushed. To be honest I thnk I would have got better A levels there than I did in state because it pushed me, there were people there who were far more intelligent than me and competition was tough. In state I coasted, did the minimum and got mostly Bs come exams.
My own fault of course, it could be argued that having spent the preceeding years in private I didn't know how to push myself.
Anyway, loaning out teachers will not make a blind bit of difference, the state sector has some fantastic teachers and it is demoralising for them to be told that they are not good enough and that provate teachers need to be borrowed to make up for that. A state school in its very nature has a far greater mix of socioeconomic backgrounds and learning abilities which will all influence results.
Maybe it is about the whole charitable status thing. My old school offered assisted places off its own back after the government stopped them, it also encouraged its pupils to participate in childrens' university. Is that enough? I don't know but all this just seems to be yet another stupid government iniciative.