All Class Notebooks are stored in your OneDrive, so that's where to delete them.
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Sign in to Office.com with your school email and password.
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Select OneDrive.
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Select the Class Notebooks folder.
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Select the checkbox next to the notebook you’d like to delete.
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Select Delete.
Note: Only owners of notebooks can delete them. You won't be able to delete notebooks that were shared with you.
Source: Microsoft