I attended a very small Christian school. In terms of teaching the particular brand of Christianity we practiced, it was worthwhile.
In terms of teaching all other forms of religious expression, it did a lot of harm. We had a Worldviews class in which we were taught, really, that everyone else out there was actively fighting for Satan in one tricky way or another. In terms of dealing with marginalized groups... I'm ashamed. We were shown a video in which gay men with HIV/AIDS were dying on gurneys. I hadn't ever seen a naked woman, but was shown essentially pornographic pictures of women with various venereal diseases.
It also lacked a little bit in the sciences; for instance, I spent all of my "math classes" alone in the library with a textbook, because the one math teacher we had admitted she couldn't teach any concepts beyond a certain point.
Among all of my friends, I was one of the few who took the religious aspects seriously. It was a shock to find out, years later, that nearly everyone was having sex with somebody. This was a serious blow to my faith.
Everyone is surrounded by sinners. I think blatant sinners often do less harm than hypocrites. I'd much rather be friends with a tax collector than high priest.
Sorry. I'll do the survey.