How would it work in practice?
The teachers there would still be able to teach, they're not going to get sacked.
So to make the maths easy, around 7% of children go to private schools.
So if you took 200 children, 14 of them could be in a private school class, and the other 186 in state schools.
14 have one teacher.
The others would be in classes of 30+, so that's 6 classes of 31 - as an example.
Altogether there are 7 teachers. so the new schools would all have classes of 27 (or occasionally 28) instead of 31 without employing anyone else or changing any setups (bar moving a few chairs and desks into the previously private school classes).
The facilities would be shared between all of the pupils, so every one of those 7 classes would have access to the private schools gigantic playing fields, full range of sports equipment, up-to-date computing equipment, music facilities - many have art studios with kilns, proper theatre spaces, recording studios, large libraries, SEN departments etc. Some of those things could be moved around to the other schools, or pupils could travel to the private school (using their minibuses/coaches) to use them regularly.
Parents currently paying school fees would no longer have to, yet their children would still have full access to all of those facilities (just have to share them a bit more).
Parents previously paying school fees would suddenly be aware of things like the narrow nature of the national curriculum, would make a fuss to ensure that languages, arts, public speaking, lateral thinking and subjects like psychology or philosophy A-levels are still available to what will now be the whole of the nation.
Statistically, children with the potential to cure cancers are more likely to come from that 93% previously underfunded than a narrow 7%, so it's more likely that society will benefit. Some future genius in science or technology, or a great musician or even politician will no longer be hampered by having to share a textbook between 3, with not enough equipment or supervision to do more than the absolute necessary to pass an exam.
It is monstrous that children are not all given equal access to equipment, resources, teachers' attention etc. Can you imagine going to a playgroup where some toddlers were given only a few chipped bricks and a couple of toys to be shared between a large group of them, while the others were fenced off into a large space with tables filled with jigsaws, clay, brand new bricks, were surrounded by staff to sing songs and learn the alphabet?
It would feel wrong.
And it is still wrong, until a person has become an adult, to limit their education based on whether their parents can (no matter with what sacrifices) afford to send them somewhere radically better-funded.
93% of the population should be behind this idea, and as they'll be saving a load of money every year for the same facilities, so should most of the 7% too.