For curriculum purposes, I'd go social. A FB-ish app doesn't involve huge amounts of wizardry, but it includes one of almost everything and it builds naturally.

For curriculum purposes, I'd go social. A FB-ish app doesn't involve huge amounts of wizardry, but it includes one of almost everything and it builds naturally. Start with static content (landing page, T&C, privacy, etc.) to demonstrate basic HTML document structure, CSS, and the value of server-side includes (DRY for HTML). Progress to signup/authentiction for a quick intro to database use and security. Build on that with user profiles, and add searching to create friends networks. There are a number of messaging contexts (to use FB terms, status, wall, and private messages) and response types (comments and message threads) and read/unread item management, all of which can be initially done with forms and server-side page refreshes. Finally, you can enhance with client-side activity (AJAX/AJAJ). Perhaps best of all, though, an app like that, poorly done, can lead to scalability problems even with a couple of dozen users on the sort of machine that is likely to be pressed into service as a server in a classroom setting (assuming, that is, that the students have a sandboxed server they can safely break now and then).