Do we have to administer the field tests to all students on the same day?

No, the CBT field test does not need to be administered on the same day to every student in the field-tested grade. Schools can choose to administer the field test on as many days as they choose, within the field test window. 

The computer-based field tests are to be administered May 21–June 8, 2018. The 2018 Grades 3–8 English Language Arts and Mathematics Field Tests will consist of one session. Schools have been assigned a specific grade for the English Language Arts Field Test or a specific grade for the Mathematics Field Test. The field tests can be administered to groups or classes of students in a single class period of approximately 40 minutes.

The above information is available on page 4 of the CBT SAFT School Administrator's Manual

The flexibility for schools to schedule CBT field tests differs from operational CBT scheduling where all students within a grade level are required to be tested on the same session on consecutive days. The following information was provided to Principals in their field testing assignment letters sent by Questar in March 2018: "The field test does not need to be administered on the same day to every student in the field-tested grade. Schools can select to administer the field test on as many days as they choose within the field test window."