Not just selection bias. Look up how some of them are selected, through lottery. Involved parents sign up kids for the lottery and they end up going to either

Not just selection bias. Look up how some of them are selected, through lottery. Involved parents sign up kids for the lottery and they end up going to either school randomly. The kids getting into charter schools show more improvement than those going to public schools. Both groups probably outperform their peers in their individual schools. You could argue that charters have a selection bias for more passionate teachers and staff, but that's kind of proving the point of the charter schools.