A great site to check out for blogging with students is www.think.com that appears to be very safe for students that comes with permission slips for the parents.
It sounds like this blogging software can be monitored by other adults in the building so, if you have a huge number of students using this blog you can recruit some help to monitor the responses. Get the librarian, other teachers, and the counselor to help out.
Now teachers are using blogs. Beth did a great job giving us an overview of blogging and the applications for not only our students but personal and professional growth as well.