How old is your granddaughter? I find it hard to believe that parents with very much current knowledge of the public schools would prefer any of them to a private Christian school in most circumstances, no matter their religious convictions or lack thereof. If someone had made me that offer for my kids, I would have taken it in a heartbeat for my youngest. My older girl was in advanced classes (which are both better scholastically, and tend to get the better teachers), and played in the school's top band (which wins awards all over the country), so I'm not sure she would have wanted to leave all that. But, I think my youngest would have been MUCH better off.
However, the way to do it is to start them there from the very beginning, in kindergarten. After a certain age (different for different kids) it gets very hard to get them to leave their friends.
She's thirteen. They go to church but it's a Presbyterian church and I don't know if the pastor is born again or not. He's a lawyer! Sounds worse and worse, doesn't it? He seems very nice but I want my grandkids to go to a church where they learn how to pray and have a personal relationship with Jesus. She has asked Jesus to come into her heart but I don't know how much she's being taught, either at home or in church. I'm not there.