Of course, private schools can select their students based on any criteria, just as consumers of a private school education make choices based on anything (religion, gender, sports program, etc.). But, to say merit is not a main driver is not correct. I don't think a private school will survive with the slogan "We're just as lame as the pubic school down the street. Give us another $15,000/year." No, they generally offer something else to garner a critical mass of customers willing to pay the extra price. And most of the time it's a better education.
Likewise, most parents have to send their kids to public schools because their money is automatically sent there. They have no extra to also buy a private school education. They have to accept whatever quality of education the government and unions provide. If they could shop around in a competitive market (and take their money with them), the more intelligent non-wealthy kids could get a better education.
