Hey Kathryn
I am curious if you have made any progress with this issue? I have the exact same need for a client of mine. I have also tried to create a custom role , first based on the student archetype with no success, one with the teacher archetype and one from scratch, also no luck.
In all instances I was able to grant editing rights to a the role (turn editing on) while being able to lock the role down to certain content files via groups. here is where I ran into a problem; the edit dropdown menu on all content displayed, but was not active, if that makes sense
FYI I am on v.3.3.1 and adaptable theme