|Transfroobium, as far as you know|
In a system where school and teacher performance is based mainly on test scores, with a smattering of other data like attendance figures, the product of a public school is data, and students are not customers at all-- they're the raw material. And the free market is not kind to raw materials.
Let's say my widget factory uses transfroobium to make the widgets. I am not just going to accept any source of transfroobium for my supplies.
There is, for instance, one sort of transfroobium that comes out of mines in the East that comes laced with certain other elements; it's perfectly good for widgets in the end, but first I would have to run it through extra processes. Those extra processes cost me money, so I'm not going to use that form of transfroobium.
Transfroobium from the West tends to be a little less concentrated, so it makes widgets of slightly lower quality. I may decide that it's an economical choice, but if my widget quality drops below a certain point, it may hurt my business and I'll have to stop using western transfroobium.
And northern transfroobium actually produces excellent widgets, but it is so terribly expensive to mine and transport. Can't have that. And there's some transfroobium that comes from other regions that I can work with-- provided the regulators are willing to just kind of look the other way when I employ some not-entirely-ethical methods of processing it.
What I really want is that southern transfroobium-- concentrated and unmixed with baser matter, it makes high quality product without too much investment on my part. I can get the results I want and still maintain a hefty profit margin, which is of course the main factor I have to consider here. I mean, come on-- I'm not making widgets as a public service. I've got a business to run here.
I'm also not going to deal in other companies' cast-off materials, transfroobium that has already been partially processed in another plant. I want to process my raw materials from scratch or not at all-- I'm not trying to work with someone else's half-finished discards.
The one thing I absolutely can't do is work under a government order that says I have to use whatever transfroobium is at hand. That would be like telling Ford they have to make cars out of bananas, or telling US Steel they have to make steel out of whatever they dig up in their CEO's back yard.
The most fundamental right, the most basic principal of any business in a free market, is that we get to decide what raw materials we are going to use to create our product.
I am not arguing that free market businesses are inherently evil. But I am arguing that in a free market education system, particularly one stapled to a test-centered accountability system, the market forces will inexorably push schools to be selective about their raw materials, about the students that they choose to allow through their front door.