The forum tracking shows the difference between read and not read posts.
In order to activate this functionality:
1. Turn editing on the classroom and go to the forum configuration. Then, in the Subscription and traking section, select Optional from the Read tracking menu.
2. Also, each user, has to access to their Preferences menu.
In the Forum tracking section > Forum tracking, the user has to select Yes: highlight new posts form me option and save the changes.