I co-founded an educational social network called Elgg.net in 2004 (which later became Eduspaces.net when Elgg's scope grew from education into an open source social networking platform for all), and I've been watching this space carefully. There have been quite a few services which have been and gone that have followed the model you've outlined, but that doesn't mean you won't be able to build something that works.
The biggest deal, in my opinion, is always this: find a way to make it work for the first user. Build it around existing educational resources, or tools for an individual to learn, and then let the community enhance that initial offering. Otherwise you've got one hell of an uphill battle ahead of you. (Pertinent examples include Flickr and Delicious which both allow you to store and share resources, but are so much more when millions of people are also participating.)