I can't remember where I read it but you are correct. Charter schools do end up taking money from the same budget that public schools get money from. From what I understand (and someone can correct me if I'm wrong), the rational is supposed to be that it will force the public schools to improve because then they have to compete for students with the charter schools. Which ends up being a losing battle for the public schools in the long run, since charter schools are privately run. They can pick whatever students they want and they can also decide to close if their parent organization decides it's no longer financially beneficial to stay open. The Zuckerberg school closed because the org wanted to pivot into a different area.