The Student Parent Directory (New Times Roman) report sorts all users alphabetically, last name first. This also includes students with Divorced Families. The parent whose name comes before the other alphabetically will be displayed at the top.

In order to make a user come first, add any special character in front of a users last name. 

First Name: James
Last Name: (Jim) Smith

Will come before:
First Name: Adam
Last Name: Abelle

Because of the '(' character.