If, as you suggest, the kids who move in do much better and the kids who are displaced do slightly worse, that does indeed sound like a very good deal economically.
I don't think that was an implication of the parent comment. As an example, Consider the toy zero-sum model where there are ten fixed outcomes. A kid is moved from outcome 10 to outcome 2, doing much better, but every kid from 2-9 is bumped down by one. By construction, there is no change in overall outcomes, but the oversimplified claims in the article that parent is complaining about would chalk that up as a huge +8 gain, by only checking one side of the ledger.
As the parent and other sibling comments point out, the real advantage may come from positive-sum effects: reduxed inequality, a more efficient market from a leveled playing field, etc. But that doesn't change the fact that the math being used is too incomplete to be held up as an economic efficiency argument, the way the article does.
