Policies aiming to improve educational outcomes are typically based on academic testing data. However, such data only reflect the performance of students who completed the tests. It is possible that students who were absent have shared characteristics. The proportion of students absent out of all eligible school students and whether they have shared characteristics has not been investigated, as this is only possible through the use of linked administrative data. Participants were born in Western Australia in 1994 and 1995, and their birth records were linked to participation status in the Year 9 academic tests to determine the proportion of students who were absent. Logistic regression was conducted to investigate characteristics which predicted absence on test day. A proportion of non-Aboriginal (3.2%) and Aboriginal students (21.9%) were absent on test day. Risk factors which predicted the absence included contact with Child Protection and Family Services, history of maternal mental health problems, and fathers aged below 20 years at the time of their child's birth. A significant proportion of students was absent and therefore not represented in academic achievement information. These students were more likely to have experienced adverse events and therefore are not randomly absent. As these data are typically used to inform policies which aim to improve educational outcomes, they may lack the necessary information to adequately address the complex needs of students who are absent on test day. However, findings suggest that service providers, as well as schools may play an important role in encouraging participation in school.