A post in weblogg-ed has pointed to an article in the Wall Street Journal on the use of blogs in classrooms. One of the major issues about this seems to be the lack of control over the content. 

In an earlier statement the SQA Computing Blog said that exactly this could be a big advantage of 21Publish as it provides full control for teachers over student's blogs. And of course this has been one of the cases we had in mind when we launched the service. Not only would a class have blogs for each student but also through the portal pages would have a customized homepage of their own. Also as each student's blog comes with at least 5MB webspace it should be of great value for the student compared to personal blogs on other services.

Of course student who are in to blogging  will have separate private blogs in addition to that where content cannot (and in my opinion also should not) be controlled. But teachers can guide on the use of blogs and how to make the most out of blogs.