If I remember right, it's also due to the fact that the type of parent that will spend the money and time to enroll their child in a charter school is much more

If I remember right, it's also due to the fact that the type of parent that will spend the money and time to enroll their child in a charter school is much more likely to be the type of parent that takes an active role in their children's education. There's an argument that a charter school's real advantage is that it attracts a higher proportion of parents who want to push their kids for achievement and have the resources to do it as well.