How do you do it?
In our town there are a couple of private schools. The land they are on is maybe 6x the size of that of the state schools. They have their own theatre, sports facilities, huge libraries etc etc.
I suppose to make all schools state schools then you could open all their facilities to all the other schools in the area, share out the books and the equipment, the computers, etc. or perhaps
children from other schools in the region would use that one as a kind of hub of knowledge - use its fleet of minibuses to take classes there for sports activities, swimming, library use, drama productions, etc.
Local children could go there still, but the facilities would be properly shared with all other schools. Class sizes could go up to 25 to allow for the state schools with 35 children in them to get smaller classes.
Then everyone is in a smaller class, everyone has access to top-class facilities and up to date technology, and everyone benefits - as in Finland.
Alternatively, if that's all too complicated, you could build a brand new state school on part of the land, sell of the rest for development, and with the huge income from that, fund that new school and all the others in the area.