The worst schools, the kind that people convert, move, or pay to avoid, are a product of bad parenting.
The best schools are often purely a product of good parenting (i.e. if you compared the management, etc. with the bad schools, they wouldn't be any better).
There are precious few schools that transcend their raw materials (i.e. the children and their home environments).
This is not primarily an issue of school financing.
Education spending rose 71% in real terms over the previous government (1997-2010). Where did this money go? Mostly into higher wages.
Obviously good news for teachers, but 'output' absolutely did not improve by 71% over this period.
Private school fees increased at a pretty much identical rate btw, I think reflecting competition for resources from the state sector (i.e. higher wages).
Combined of course with the withdrawal of assisted places scheme in 1997, private education is now reserved for the children of country's wealthiest families, i.e. the offspring of the likes of Nick Clegg, Dianne Abbot, George Osborne.
The previous government's policy of ruinous house price inflation and unprecedented levels of immigration also resulted in normal parents being priced out of the catchments of many good state schools.
If you are not rich, with politicians having priced parents out of both housing and private education, the only option in many areas is to quite literally pray (preferably from before birth).
Given how shitty this situation is, the idea of giving politicians any more money to supposedly improve education is laughable. It won't happen.
If you increase spending, wages will go up, there will be more building projects, but will actual educational outcomes (i.e. whether people can read and write at a functional level, not bullshit inflated GCSE pass rates) improve? Probably not.