hi brian,
we can add students only to those class notebooks created by ourselves. if not, when we go to the
onenote class notebook creator and click add or remove students, we will get this prompt even if we are co-teachers:

given this, i would suggest you mail to *** Email address is removed for privacy *** to submit feedback so we can continue to make the tool and onenote even better for use in the classroom.
thanks for your understanding.
update on 2016/1/14
the info above is outdated. we recently shipped updates to allow co-teachers to add/remove students for new class notebooks. see the post below and the blog at
https://blogs.office.com/2016/01/05/ring-in-the-new-year-with-onenote-class-and-staff-notebook-updates/ for more info.
regards,
james