I'm working in a primary-aged school, so this may not suit your particular context, but we made things fairly simple, along the lines of 'you behave online as you would in the classroom'. Here's what we say:

• You concentrate on the work! So our focus is on this to help us to learn, although we can have some fun occasionally too;
• You treat everyone with respect: you really mustn't write anything which might upset anyone else, or that you wouldn't mind seeing written about you yourself;
• You join in, so post entries to the glossary, reply to other people's questions etc.;
  
• You ask for help if you're not sure about something, or are challenged - the homework discussion is a good place for this;
• You respect others' privacy, so don't use someone else's username, or let other people know your password; and
  
• You look after shared resources, so try not to break anything!
  

We have one for a K-12 school at the link below...feel free to use anything you may find useful.

http://www.modellabschool.com/online/mod/resource/view.php?id=3#internet

Steve

In our high school district, every teacher who wants to use moodle must first follow the attached lesson plan.  Among other things, it requires students to read the District's computer/network AUP (which does not make specific mention of moodle). 

What is AUP?

Rich