Disposable cameras are OK, but this is where a cheap digital camera might be better. Not only are the images easier to get, they are also easier to share, either with the group or the world. It is also the medium of their generation. As fun as we may find the photochemical process, it does not alway translate--and is the project of exploring and sharing their world (I hate "self-expression") or is it to learn a photographic process? If it is the former, then a digital camera is really going to free them to explore photography as a creative medium without being bogged down with technical problem, or at least fewer technical problems.
You will need a frame work in which to focus the kids and get them to work. The trick is to make it flexible so you can adjust with the reality of the student's ability--since this is your first time, it is easy to under- or overestimate what they can do. Having an overarching goal and a way to build on skills to get there are important. I have found there are no tricks to education, just well thought out planning and the ability to adjust as the situation demands.
I do quite a bit of instruction in imaging, desktop publishing, and design at the undergraduate level with folks outside those fields including teenagers. Children get even harder are their ability to think abstractly is developing and at some young ages, like 6 or so, then have no way to think abstractly.