Awesome - glad you're enjoying it! One thing to note: If you make changes to the code and then hit play again right now the user's changes are lost (making sure that playback will continue to work). We definitely want to persist those changes in some way, automatically, although in the worst case the user can always make some changes and hit 'Save' and they'll be saved to their own account.
As long as you're here, I'd like to bring to your attention a UX issue that might deeply impact first impressions. If you visit the site for the first time and click the play button, there's a good chance that, immediately following this click, the mouse cursor will stray into the code area.
This causes the code to turn white, and so the viewer just sits there listening to the lesson and watching things change in the running-program frame, oblivious to the fact that the code is invisibly changing, too (one of the coolest parts of the product).
You might want to unfade the code and get rid of the "Pause" button if the cursor hasn't moved in a while, to ensure first-time visitors don't miss the awesome code-to-audio syncing.
Other than that, I'm super impressed with the site.
My ideal scenario would be to have the student's code saved in a sideboard, while the teacher's code would take over the forefront. Sort of like a really minimalist branch feature.
If you could make it work with the layout, it would be really cool to have the student's code window physically move off to the side, indicating that the teacher was taking over, but their code would still be there whenever they wanted to get back to it.
