The Login Message block is a pretty slick way to differentiate notifications to students based on their login statistics. Each can be customized to fit your needs.
Messages are each displayed in a regular block and are triggered by login.
- Before login
- First time
- 1 day since last login
- 1 week since last login
- more than one month from the last login
Might be a good way to promote model student behavior. Check it out and download it here: https://moodle.org/plugins/view.php?plugin=block_login_info. The block is developed and maintained by Hiroshi Honda and is available for Moodle 2.7.