My school system - City of Salem Schools, VA - has undergone a lengthy process to determine what types of social networking should be available on our system's network. Until recently, all social networks were blocked by our filter.
After much discussion and exploration, it was decided that social networking would be open for all faculty members. Faculty members would be treated as professionals who are able to use social networking appropriately within the work environment. (Our Barracuda filter has made it possible for us to open up certain sites for a specific group within our system.)
We also decided that access to social networking in general is not necessary for students within a school setting. In fact, it probably could lead to more harm than good. However, social networking does have educational value if used properly. Therefore, we decided that Ning would be the one social network available for use by students. Teachers have been encouraged to create Nings for use in the classroom but to follow certain guidelines to make sure that Nings can be used in a manner that maximizes safety and educational value at the same time.
If you're interested in using Ning in your school system, you might be interested in checking out the guidelines that we are using. Here they are: Ning Guidelines
I'd love to hear about anyone other guidelines that any of you have created.