Do you think of learning management systems (http://en.wikipedia.org/wiki/List_of_learning_management_sys...), or do you think about it being open to all?

I know some of those systems has the option for fora, and my experiences with those has just been good. If you're looking for good ways to work with an open-for-all system, I suggest looking at the most used fora in e.g. mathematics and physics, and then take some ideas from there.

A problem I suppose you might encounter is that most schoolkids use this networking site just to solve their homework: At least that's what they do now. If you could somehow change that problem into a way of making them interested in learning how one solve those problems (instead of making other solve them), then you've got a great idea.