Private school == Massive selection bias. Get everyone who cares about educating their children and send them all to the one school. Then post-hoc rationalise

Private school == Massive selection bias. Get everyone who cares about educating their children and send them all to the one school. Then post-hoc rationalise that it was the schools amazing teachers/facilities/whatever that caused the child's success. Nothing at all to do with being part of a family that cares about education.