Many times I have heard conservatives talk about K12 education as if it were a market, especially when the topic of school choice comes up.
I am completely in favor of school choice, but there are some important ways in which the market idea fails. If we want to talk about how to improve the situation, it might be useful to explore those ways.
First, education is mandated. In that way, it is like Obamacare. Parents cannot just opt out. If you want education to work like the free market, you have to let parents opt out, and you have to accept that some parents won't be able to afford to send their kids to school. Some kids won't get an education, just like not everyone can or wants to buy a Cadillac.
Second, public funding plays an essential role. As soon as you introduce public funding, including school choice vouchers, you are again not really dealing with a free market situation. However, if funding were attached to students, and students / families could choose which schools to attend, wouldn't that be analogous to a free market situation? Not as things stand now, because ...
Third, public and private schools do not follow the same regulations. If you really want to see how they stack up against each other, they need to follow the same rules. If we want improvement, we would let public schools act like private schools, meaning they don't have to take any given student, nor keep problem students, nor keep problem teachers. Here again, if you let every school decide not to take some students, you will end up with some students not getting any education.
How much we really want K12 education to be a free market depends in part on how determined we are to offer every child an education. The more like a free market we want it, the less we can insist that every child have the chance at an education.
So why am I in favor of school choice? Because we are stuck with a system that doesn't work for a lot of students. Until we pretty thoroughly overhaul our system, school choice is the best we can do.