There are a wide variety of REASONS to blog with students in the classroom, and many options in terms of platforms to use. The biggest reason TO blog is: The more we write, the better writers we become. There are lots more!
Our classroom blog this term is a commercial (not free) blog hosted by 21classes.com. Our class blog (linked under "Resources" too) is untcic.21classes.com.
Two ways to define or categorize classroom blogs is by purpose.
Kidblog.org (free)
Class Blogmeister (free)
21classes (free for just 10 students, then $9 per month)