Educators and owners of Class Notebooks are not advised to share them using the Share feature in the OneNote for the web or other OneNote apps. Doing so will only grant access to the Welcome page of the notebook.
For best results, add students and teachers to Class Notebooks following these steps.
Check notebook membership
For students, use Add students to a Class Notebook in OneNote. For educators, use Add or remove educators in OneNote Class Notebook. For Staff Notebook members, use Add or remove staff members in Staff Notebook for OneNote.