Once a user is added you cannot change their relationship. You will need to remove the relationship and add it back in Core.
  1. Go to Core > Users > Edit user profile data
  2. Enter a name and search for the student.
  3. Under Relationships, select Delete next to the user that you wish to change.
  4. Then select Add to add the user back.
  5. Choose the correct relation type and give them the desired rights.
  6. Click Next 
  7. Click Search for an existing user and search for the user.