Thanks for your help and sorry for my delay in replying.
I checked whether the teacher can access her OneDrive, and yes, she can. When I first tried to access her OneDrive (logged into Office 365 as her, of course--and on her laptop), I was unable and had to go through the hijinx I described earlier in this thread.
I believe I mentioned that it's been common for us to be initially denied access to OneDrive over the past couple months despite our being already logged into Office 365 and viewing our email through the Office 365 Outlook app. However, I did finally gain
access to her OneDrive--using the hit-and-miss steps described above in my December 15 post. Despite this, however, I still could not create a Class Notebook. Same failure as initially described. So this is a long-winded answer to your question in #4 of
your reply above: Yes, that is what I mean.
Here's the screenshot of the error message you requested:
Wait. Cancel that. I just remembered that, as I mentioned in the final paragraph of my initial post above, trying to insert an image will freeze my screen and result in the old do-over for me. I copied all this text just in case, so I'll try, but I imagine
I'll be redoing this post.

Nope! Good news! Adding an image no longer freezes my window. Well things are looking up. :-)
To answer your question from #2 in your reply above, the error occurs at the end of the section titled "Finalize Your Class Notebook", right after I click the "Create" button.
Finally, I sent you the teacher's username via the private message link you provided.