That's a big assumption.
Where do Missionaries go to preach the Gospel? To the First Baptist Church or to some a country full of atheists and non-Christians?
If the parents are that anti-Christian as to undermine all our teachings, they would not consider enrolling the child in our Christian school in the first place.
This mother may be a "do as I say and not as I do" type of parent. That is not ideal as a child should be taught by example. However, if the parent is willing to give her child a Christian education, the child should be given that opportunity.
I cannot imagine Jesus turning a child away for the sins of the parent.
I can honestly say that I wouldn't trade the cultural experiences that helped shape me through early adolescence for anything else I could have done at the time. Still, I have to admit that having parents who are non-believers decreases the chance that children will accept and spread the gospel.
Another thing that gets glossed over here, is what could happen if the stripper mom had drug addicts for friends. Could the child find drugs and bring them to school? What stories could she tell the other students? What psychological pathologies might she be at risk for? How much long-term trouble would the school be in for, e.g., if they had to consider calling child protection services? Lots of questions come to my mind that have not even been posed, much less answered.