since nobody answers...

a small idea from me

on www.openprocessing.org it is (Beta) possible to edit a sketch on the web site and run and store it there: Link

Now, if you would create a new fake user "my cool course 2013" or so and hand the user name and password out to everybody - it might work that everyone can work on it.

• Since this is a hack, when 2 users work on the sketch the same time, changes would get lost I guess
• Since it is Beta it is not too stable either ...

Chrisir     

Well, I know this site but that's not what I asked about. I don't want someone to modify the code. I meant online 

I see

Like a multiplayer game over a network?

Ah, I totally interpreted the request the same way than Chrisir...
If you use Processing.js, I think you can use WebSockets for this kind of purpose. I don't know much more, but there are lot of tutorials on the new features of HTML5.