I feel that honestly, the choice of sending a child into a public school or to a Christian school, or possibly even home-schooling them, all really depends on the child. As far as exposure to non-believers in public school, I haven't worried about this with my son much. He always comes to me with questions, trusts what I teach him, we read scripture together and pray together about whatever he may bring before me. Last year he had heard a girl, in the grade above him, use the exclamation "OMG!" now, being the little boy he is, he came home and asked me what it stood for. When I explained the meaning and that she had actually unknowingly used an abbreviation for a statement that used the Lord's name in vain, he was appalled. The very next day he told the girl (a Catholic, Hispanic girl) what that saying meant while her and her friends were standing in the hall way. All of the girls were surprised and none of them have used the exclamation again. My pastor's two children are wonderful examples of two Christian high schoolers that are a wonderful example for their peers (both believers and non-believers).
However, some children even when raised in a Christian home can falter. I had a friend when I was in high school. Her mother was a wonderful, kind and involved Christian woman raising her two daughters. My friend knew and had been taught from a very young age to be a believer and how to live a Christian life. They moved to a neighboring town with a bigger high school. She started using drugs, dating a guy that was high ranking in the town's toughest gang, and just after we graduated from high-school I lost track of her and to this day have no idea what happened to her or where she is. She is an example of a kid that needed exposure to other Christian kids her age instead of being left to be led astray by exposure to kids of "the world".
Thus, every parent needs to be observant of their child and decide if the exposures at their school has a negative affect on the child or not. They can then decide for their own child what school is more appropriate for them and their child.