J.S.
HI!
I work for a company that teaches through Blackboard and moodle. I myself teach 7-12 math through BB. Moodle is suppposed to be very similar. This is definately not a system where bulling is a common problem. In fact, many of our students come to us to avoid bullying. The nice thing about these systems is that they are closely monitored and any problems that do pop up are traceable because the company I work for doles out the passwords and login information. We have strict policies about inappropriate messages and in my two years doing BB any time an issue has popped up it has been taken care of swiftly. I do not know your school's plans with the system but my experiences have been positive with the similar format.
Good Luck.
JB