I've been searching the web and not yet found a wiki that is embedable, however many wikis offer rss feeds, you could for example add an rss feed of recently changed pages on your classroom 2 page. Clicking on the links would take readers to the wiki in order to continue reading or editing.
I recently found a way to put the wiki inside of the NING. I created a new tab, which then I linked to my wiki which was hosted outside of the ning and made sure I had a link back to the NING. While it's not truly "inside" of the Ning, it is a way to link it.