I don't know how I've missed this in my previous mentions of Posterous (here and here), but I just realized that Posterous offers a really simple group blogging system. Posterous is known for its dead simple posting system. To post to your Posterous blog you simply send an email to Posterous. To create a group blog simply sign into your Posterous blog and add the email addresses of those people who you would like to have contribute to your blog. See the screen capture below for more information.
Applications for Education
Posterous could be a great option for creating a classroom blog to which all of your students can contribute. In order to post to your classroom blog created with Posterous all your students need to do is send an email. They don't have to remember any usernames or passwords other than their email. Posting through email also ensures that you won't have any students accidentally deleting another student's contribution.
Here are some related items that may be of interest to you:
TypePad Micro Offers an Easy Way to Start Blogging
Gaga Post - Group Blogging
Soup.io A Simple Yet Feature-Packed Blogging Platform