I taught for a few years, and I met many parents and students. The old adage is right: judge the tree by its fruit. Voucher-funded charter schools don't even have to cherry-pick students to surpass public schools: the biggest factor is that parents have to make a conscious decision to send a child to a charter school over the default public school. The worst-performing students belong to parents who will not put in the effort to choose a school instead of sending their kids to the default public school. There is a direct correlation between the parents' (or more likely parent's or grandmother's) lack of involvement in the child's education on the child's performance in school. The easiest filter to separate the bottom of the population from the average and top is to provide a choice of schools, because the lowest segments simply will not make any choice.