I have used many blogging platforms for students--the safest, easiest platform I have found is http://www.teacherblogit.com
This site allows teachers to quickly put together a classroom of bloggers that can be kept private or public. Students can use their mobile phones to access the site as well. It doesn't get any easier--and it is not a wordpress platform, so there are not added complications to posting and commenting (students can even add their own avatar). I wouldn't use any other blogging site with my class.