If they want that sticker to say on school textbooks, I think they have to have the corresponding sticker placed on bibles in their churches:

"This Bible contains material on God and faith. The existence of God is a theory, not a fact. This material should be approached with an open mind, studied carefully, and critically considered."

I mean, if the churches expect to have influence over the material that is taught in schools, then they should be willing to accept influence from outside, as well. It's only fair.