If you do not see a student as added to Class Notebook, they may not have been added properly. Please follow these steps to properly add students to Class Notebook: Add students to a Class Notebook in OneNote
If you are using School Data Sync and newly added students are not showing in Class Notebook, please follow these steps:
Open Microsoft Teams, then navigate to the Class team where the Class Notebook is located.
Select the Class Notebook tab. This will trigger provisioning so that new members are seen in the notebook.
Note: Whenever student/faculty members are added/removed in School Data Sync, the above steps must be completed in order for the changes to be seen in Class Notebook.