This is simply common sense. Kids are society's investment in the future, and as in all areas a wise man invests in the most promising prospects. We invest the most effort and resources in those students we feel are going to give the most back to society. Having said this, of course, the current model isn't fit for purpose. The least academic kids should be steered towards vocational careers and allowed to leave school early to go into apprenticeships. And,actually, many of them would then end up earning a much better salary than their teachers: and probably making a much more valuable contribution to society than many teachers.