I do not know much about this free tool, but I was just notified that Edmodo has launched.

Edmodo is a private microblogging platform developed for use in the classroom by teachers and students. Edmodo provides teachers and students the ability to share notes, links, and files to foster communication inside and outside of the classroom. Teachers also have the ability to post alerts, assignments, and events to share with their students.

Edmodo looks like a very useful tool, has a clean interface (feels a bit Drupaly), and could be an excellent tool for classroom teachers looking for a private blogging option. If anyone starts using it with their students, please let me know.