Teachers can set student avatars, instead of behavior characters. When this mode is enabled, milestones will not be shown at all. Here's how.

1. Open the class from the left menu.
2. Click on the Behavior tab from the Updates icon.
3. Click on the More... link.
4. Choose Settings.
5. Under Theme, click the option.
6. Choose Student Avatars.
7. Your behavior page now shows student avatars, instead of characters.

PS: When Student Avatars is enabled, class goals are disabled.