This is because the Students' information is not set to publish to the Teacher Role.
To fix this navigate to:
  1. From the menu in the top left select Core
  2. Locate the user with the People Finder
  3. Click on the Settings tab
  4. Under Privacy Settings "Include my name and only the specific information selected below to those specific constituent types." Select the Teacher Tab
  5. Mark off the items you want the Teacher to see
OR

you can reset the Profile Publish fields for ALL Students to Teachers in case you had a bunch of Students change this. 
Note: Resetting Publish Fields for ALL users will NOT override an individual's selection to "Include my name, but do not include any other information about me." To reset access, the profile fields have to be manually set through the user's individual publish settings.
  1. Core > Security > Profile Access
  2. Select the tab for Profile Publish Access
  3. Leave Student in the dropdown and select Student to Teacher in the Manage Publish Fields link
  4. Select Edit in the upper right
  5. Select the Radio Button for Reset Publish Fields for ALL Users
  6. Click Save and Confirm
  7. Click Confirm
Note: these changes can take 24 hours to refresh, so the change may not take effect immediately.