There are multiple ways of copying a test:
Copy the test within the same class
Go to your class, to the tab "Tests". Click on the copy icon.
Choose the name for the copy of your test and click on Save.
Copy the test and have the copy in another class
Follow the same steps as above, and in the new window choose the class where you want to have your copy:
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Copy all your tests from last year's class into the new class
It's also possible to make a copy of the full class including the users, the course material and all the tests. This is a good option if you are preparing your classes for the next academic year. Learn how to do that here.