Hi Gerald,
I discovered the same problem. And I don't know why the navigation block is visible for teachers while in edit mode. I also set view and edit block permission for the teacher's role to allow - no success.
If you can allow teachers the course creator role, they will see the navigation block. If this is not suitable for you, you may have to create a new role based on the course creator role and prevent some or most of the course creator's additional capability.
With kind regards,
Hartmut
