Private unaccredited religious education is really messed up. Two of the main reasons for sending ones children to such a school are 1. to isolate and indoctrinate, 2. to reform children who have been kicked out of public school. These go together well. When teaching their children how evil the outside world is they only have to point to the very troubled second group and say, "See them? That is how the world without God will make you". That is just from my experience and I do not have any studies to back it up. But here is a guy who backs up everything he says and I really like his blog. Please go check it out.