Facebook EDU: a school-managed Facebook account with parental controls. I wonder how long it will be before this happens. I like your idea, but I'm not sure how I would feel if Facebook operated it.
Damned if you do, damned if you don't; Do you want useful features rolled out on a regular basis, or you want peace of mind that the company involved is dedicated to working directly with K-12 institutions as their primary concern, without alternate revenue streams and business needs getting in the way.