When you see something other than the Gradebook appear it may be due to the teacher not actually being set as a Teacher under their employment. To check go to:
  1. Core > Users/Access > Profile
  2. Search for the teacher and click on their name
  3. Navigate to the bottom of their profile under System Information
  4. Click the Employment Button
  5. Edit into the Employment
  6. Under Employee Roles make sure Teacher is checked along with the School Levels. 
  7. Click Save & Exit
They should be able to log in and navigate to their class and access the Gradebook from here on.