This is brief as I start playing with it, but you might want to check out Zonkk, a new social networking tool which just went public.
I say tool, because rather than being a pre-set up site a la Ning, Zonkk allows you to set up your own social networking site, with a pretty hefty (and slightly confusing) set of features.
The website is currently pretty weak on details or tutorials. I hope they're working on that. But if the photos they give up sites already up and running are anything to go by (and why aren't there links so we can look at them for ideas?), this has potential for schools as a whole or individual classes.
While it's free, there are the inevitable ads on the website you create. It will cost you $15/month to get rid of those.
I'll post more details once I've had time to play with it.