I taught at a private Christian school, and we only accepted students that were from a Christian family, we usually required a letter from a pastor/ church leader for acceptance. They had to agree to support and uphold the Christian ethics of the school.
I don't think it is 'wrong' to do this as pp said, because we expect to provide a high quality, christian education, and we expect to work together as a team with parents in raising and training their children. How can we work in a team with parents who have opposing beliefs to the school? When we say there is only one God, and they say there is no God? It goes completely against the philosophy of the school.
Also, if someone is not a christian, why would they pay a lot of money to send their child to a school where they are immersed in christianity, and it is a part of their every day? So usually if parents were not christian, I felt like they wouldn't 'want' to come to our school any way, because Christianity was such a big part of what we do each day.