BuddyPress.org

I had to make the whole site private using this plugin:

http://premium.wpmudev.org/project/sitewide-privacy-options-for-wordpress-mu

There is a free version of this plugin, but we wanted to be able to override the settings by site admin only.

You cannot access any page on the site without logging in first. This works for us since we don’t want outside access to students. On rare occasions where the teacher wants the students to blog publicly, I can override the setting for the specific blogs.