Yes, but the argument goes that parents who are likely to move their kids elsewhere have kids where the cost to educate them is just a fraction of what is being spent on the average kid. Imagine a school system that spends $15k per student per year. But that is just the average. Some of the kids are only consuming $5k of resources while others require $60k per year. If the kids who are only costing $5k go to another school and take the $15k with them, they will presumably get a better education where more of the $15k will be allocated to them. Meanwhile at the school that was used to spending $60k on some students, they will presumably need to have less disparity in the amount they spend on each student.