High-quality schools can increase academic achievement among the poor (2010)
The kids were a self-selected group because attending the school required that the parents had to do something active to get their kids into this school.They try to control for this by comparing those kids to kids who lost in the lottery.  But this only tells us about kids who have engaged parents.  You cannot generalise to kids who have non-engaged parents.Having engaged parents is the single most important condition for getting a good education.Secondly, not having disruptive kids in class.And only thirdly are quality of teachers.If "free dental, medical, and mental health services" were all it took, then no school in Scandinavia should have any problem with poor kids.  But we do have problems because some parents don't care at all about their kids education.