The Teacher-Only Section is an area of your Class Notebook that is only available to Teachers of that Notebook. This is a great place to prepare or store data that you do not wish students to see.
First browse to https://www.onenote.com/classnotebook in your web browser.
Choose Manage Notebooks.
Scroll to the notebook that you want to enable the Teacher Only Section. Then click Enable Teacher-Only section group.
The page will update showing the section is enabled.
After syncing your Class Notebook you will see the new section next to the Content Library.