It works like this. Charter schools are funded by the taxpayer but not regulated by the public service. This means more competition – parents get more choice when choosing where to educate their kids. Yay!
But the Charter schools also get choice. Specifically, they get to pick and choose their students. So if you have a bright kid who is keen to learn (and cheap to educate) then they’ll get to go to the local charter school – and they take their taxpayer funding with them. But if you have a kid with a physical disability, learning disability, developmental problems, dyslexia, autism, etc, then they don’t get to go to the charter school. They get educated at the local public school, because they’re regulated by the state so they have an obligation to take those kids. They just have a lot less money and resources available to them to educate those children, because a chunk of the low maintenance, low cost students have transferred to the Charter school and taken their funding with them.
Then, at the end of the year, miraculously, the local Charter school outperforms the local public school! Even though they have the same funding! Which proves what a dazzling success they are, and how evil the teachers unions are, and how superior the free market is.