You can pull a student-level diagnostic status export (including student IDs) following the instructions here.
You also have the ability to view diagnostic status within the platform following the steps here:
Log into the i-Ready admin site.
From the landing page, select either the Reading or Math option from the Diagnostic Status page
From the Diagnostic Status page, selections will default to All Schools and the most Recent Diagnostic. Select the specific school you are determining completion for.
Select the grade
Select the View All Students link
You will be shown all students that have not completed the test at the top of the report - share these students for the grade selected with the school
Repeat for the other grades.
Find additional resources for pulling diagnostic status, below:
How to: Using Your Diagnostic Status Report as a Teacher (Video)
i-Ready Cheat Sheet Diagnostic Status District or School (PDF)