Ironically, I was just debating (informally) with some people about teaching 15 year olds last night. My opponents in the debate were rather idealistic. Mat and I… less so.
Anyway, I’ll second two existing nominations - Discworld books, and Holes.
For the Discworld books, I’d reccomend the guards series. They start light, and get more serious and deep, but remain fun and engaging. Beyond all this though, Pratchett has one amazing feature that makes his books impossible to put down: He doesn’t have chapters. This makes it amazingly easy to get into the flow of things, and very hard to put down because you can’t break up your reading by saying “I’ll just read to the end of the chapter”. Note that I’m not saying breaking a book up into chapters is an inherently bad thing, but he uses the chapterless style to amazing effect. I can’t really see them being too controversial. The only main problem I can see for a church school is the fact that the discworld is most definitely polytheistic. This is however mostly in passing, and hardly even noticable in much of the guards series.
Can you tell I like Pratchett?
Holes, on the other hand, is even less controversial. It’s well written, interesting, and fun. It’s not too long, so they won’t have time to get bored. The characters are good, and the plot is amusing (in a good way) as well as reasonably insightful. Also, the book has an extremely good movie adaption (both in terms of faithfullness to the book and quality of the movie), so the kids can compare the two.
One interesting idea would be to get them to choose a book from a list for their own reading, and read that in their own time. It gives them the ability to choose what they like (subject to restrictions). Give them a list of light reading which isn’t quite as light as to be fluff, and let them take a look at the blurbs on the back, etc. and pick a book. A good example of the type of thing I’m talking about is Iain M Banks’ Culture novels. Very light and fun, but certianly not just trashy novels. (I have a sci-fi bias though, so others can probably give you better examples - scifi may have too much of a ‘geek’ association for all of them to really get in to it).
Someone mentioned ‘To Kill a Mockingbird’. I think it’s a good suggestion, but I’d be a little cautious. I absolutely loved that book, but it seems to me to be a book that could very easily be killed by doing it in an english class. I found this to be the case with several others - e.g. Lord of the Flies. (On the other hand, some books done in english classes are beyond saving. Never speak to me about ‘Tess of the d’Urbevilles’)
Not that I’d presume to tell someone how to teach (ok, yes I would, but only because I’m arrogant), but that brings up a good point. It’s important not to over-teach. It’s important that enjoying the book comes before the actual analysis and disection. There’s nothing more dull than having to discuss and pick to pieces a book which you didn’t like.
I’ll stop ranting now. Hope some of this helped. 
