I like the Firemouth idea. Folks at FishGeeks suggested allowing the kids to choose but they'd have to do research and teenagers (I teach 14 year olds) aren't big on EXTRA work, if you know what I mean.
Firemouths will breed, and I think that would be quite educational. I am also considering a Malawi tank, as I have a trio (1 male, 2 females) of Melanochromis cyanerhabdos (Mainganos) in a 20 that could use some extra space. The only thing holding me back from that train of thought is the expense of the rocks. I may ask the kids to bring in some larger rocks from around their homes and test them for pH issues with some acid. If all is good with at least some of their rocks, I can stock the tank with those rocks. If I have to purchase rocks, then the Malawi tank isn't going to happen. The Malawi's would breed quite well, and then I'd just have to find some species to go with my extremely vicious and combative male Maingano.
I do have a soft spot for plecos so that might also be an idea. Thanks for the ideas! If you have any additional thoughts, feel free to send them my way!