I never thought of using it in this fashion, but it is strikingly obvious once you think about it. Pushing it even further, or in similar light, the workplace could also benefit from this. At our office we use SalesForce and their Chatter feature shares the similar vein of communicating within 'circles' or 'subgroups'.
This type of social networking isn't really unique to Google+ though. The big gains are going to be made when students can come together to collaborate on a Google Doc, or a professor can share documents/other files with students.