You can send a "cheer" to students, which shows up on their screen when they finish a test and gives them encouragement that their teacher is seeing the hard work they're doing.
Click on Cheer your students under the 'Other areas...' header on the left hand side of your Control Panel.
You are able to either cheer individual students on by clicking on the horn icon besides their names, or you can collectively cheer on all currently logged in students by clicking on Cheer All Students! at the bottom of the page.
Cheers appear to students in a list when they complete a test. Students can cheer each other on as well; if you want to stop this during class you can disable cheers during a homework task.