WordPress.org

Whisper Comment Reloaded is a WordPress plugin for commenters to control comment visibility for other viewers. A 'whisper' is a comment that is only displayed for targeted viewers. Viewers who are not authorised to view the whisper, will only see the replacement text instead.

This is a great plugin to use for a class blog where a classroom blog can have multiple authors where the teacher/professor may want to add private notes for the student instead of making a public comment.