You might have a group of students in your class that need to do a test, but you don't want to show the test to the whole group. Here is how you can do it:
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Create a test under Teacher-Tests, and in the test Settings make the test unavailable.
Then, go to the Tests tab again and click on the blue icon for Student Settings.
Set the test to "available" and select the students who need to see the test. If you made groups (learn here how to do that), you can search for the group name to easily find those students.
Don't forget to save, and you are all set!