There are a few ways you can unassign practice levels depending on your needs. 

Unassign the practice levels for a specific lesson:

The on-screen levels are listed at the bottom of each lesson plan under the "independent practice" section. If you'd like to lock the practice levels again for this particular lesson, scroll down to the independent practice section, and click the "Undo Assignment" button.

Unassign all levels for a class: 

Go to the class you would like to unassign, click manage class, and click "Unassign All Practice Levels"

Unnassign levels in bulk: 

The Assignments page makes it easier to unassign levels from multiple classes or to unassign specific levels.
Select the Kodable world where the levels are located, then select the section of levels. There you can see what classes have been assigned and select to unassign. 

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