Here is why.
The people of the church that support that school have the right to refuse service to you. To go to that school is not a right like a public school, they have standards. Why would I send my kid to a school to have a godly education and they allow you, a non-believer, that has different values and morals to send your kids there. I would not allow my kids to go to school with kids that don't believe in God and the blood that was shed on calvary.
The better question is..... If your a non believer why would you want your kids to go to a school with a bunch of Jesus freaks? You know they may get converted to be blood bought Christians. Wouldn't that just screw you up!
If you dont like it home school your kids. Simple answer.