If the user is not marked "List as Parent", they will not show in the printed directory document.

You can check this by:

1. In Core, go to People Finder 
2. Search for the student and click on their name
3. Open the Contact Card tab
4. Scroll down to Relationships and select Manage Relationships
5. Make sure the step-parent has List as Parent checked off
List As Parent
6. If not, select it now
7. Save