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