First you will want to make sure teachers are set as Course Content Editors. You can follow the steps in the article How can Teachers share Bulletin Board Posts etc?

As a teacher:
  1. Go to Classes > Select the course
  2. Select the Assignments tab
  3. Edit the existing assessment
  4. Click Add More at the bottom of the edit screen
  5. Expand Classes
  6. Expand the Department
  7. Click the checkbox to the left of the course you wish to share content with
  8. Click Save