I'd also imagine the parents help out a bit too. The teacher is supposed to teach, but if the parents do not support and promote the child to do his work, it makes the teacher's job all the more difficult especially when the children are young.
I'd also venture to say that those charter schools doing so well - are parents that are pushing their kids to get in their. Not all, but a good amount.
I should have said this a year ago, but maybe that was before I read Freakonomics and Super Freakonomics. Anyhow, it looks like you are right. They analyzed statistics from Chicago, and found that students in charter schools did better on average than non-charter school students. They went farther and discovered that students who applied for, but couldn't be fit into limited charter admissions also did better than average for the Chicago school district. The implication is that motivation is a major factor.