I have rarely recommended that parents place their children in them. But that doesn't mean that I am against private schools. While I may not be cheering for them, I have to defend their rights as well. I have seen private schools take kids who seemed to have no hope of doing acceptable work or forming acceptable social relationships in the public schools, and quickly motivate them and turn them around academically and socially. And I have seen those same kids return to the public schools able to function successfully.
Private schools have their place.
The battle in our legislative and judicial systems is over whether we should fund them. So allowing people freedom of choice to go to a private school instead of a public school by supporting their choice with tax money, has generally been interpreted by the Judiciary and the Congress to be a violation of the Constitution. And many support this idea because removing funding and people from public schools could lead to the deterioration and collapse of the public school system, and could lead to segregation. I think that there is another clear side to this issue. By forcing people to go to public schools and not permitting them the freedom of choice to go to religious schools, the government is squarely involved in suppressing religious rights.