It could, I suppose, be considered 'preachy'; Thomas Hughes was very clear that he was writing a defense of his views on education, and his belief in the importance of "muscular Christianity."
As for those who complain that they can't imagine the boys being interested in studying the Bible and discussing it, what they really mean is that they can't imagine themselves doing so. In today's secular society, the idea of schools incorporating Christian doctrine into their curricula is a foreign one. In such an atmosphere, however, it is not unlikely that some of the boys- Hughes makes it clear that it certainly is not all of them- actually think about the faith they are practicing, and discuss and debate it seriously.
I first read T.B.S.D. when I was about twelve, and have picked it up periodically over the years, enjoying it each time. On the surface it is an interesting story, but also has deeper levels, posing serious questions. How do you deal with bullies? What defines courage? Do you compartmentalize your faith, or does it infuse and affect all parts of your life?
Finally, it's great to read a book which treats boys as capable of thought. Their characters are flawed, but refreshingly, they don't treat adult authority figures with contempt. They also have a sense of respect for themselves, their school, and their country. It's nice not to have the boys dragged down to the lowest common denominator for a change.
* Thomas Hughes wrote a sequel, 'Tom Brown at Oxford,' which is much less well known.
* There have been several movie adaptations of T.B.S.D. I have seen the old 1940's version, which is O.K., but not great. More recently, there have been a couple of made-for-TV mini series, one of which stars Stephen Fry as the Doctor. I haven't seen these, so cannot comment on them.