Background <p>Recent findings indicate that the association between academic achievement and internalizing mental health problems or disorders has become stronger over time. It is not known if this change has been driven by specific subgroups of students. The aim of this paper was to investigate temporal changes between 2006 and 2018 in the association between academic achievement and internalizing disorders among Swedish school year 9 students (aged 16 years) across subgroups defined by students’ sex, socio-economic status, and country of birth.</p> Methods <p>Register data on all students (<i>N</i> = 1,422,487) graduating from the last year of Swedish compulsory school (school year 9) between 2006 and 2018 were used in the analyses. Achievement was measured by students’ final grade point average, and internalizing disorders were measured using data on treatment for anxiety or mood disorders. Logistic regression models were used to test for multiplicative interaction, and marginal effects for additive interaction, between achievement and time. Models were fitted separately for subgroups defined by sex, socio-economic status, and country of birth.</p> Results <p>The association between achievement and treatment for internalizing disorders became stronger over time among Swedish-born students but was largely stable (in absolute terms) or diminished (in relative terms) among immigrant students. Among Swedish-born students, the largest increase in treatment for internalizing disorders was observed in low-achieving girls. Differences in trends depending on socio-economic status were more mixed.</p> Conclusions <p>Low-achieving students and girls face multiple disadvantages in life. The disproportionate increase in internalizing disorders among low-achieving students, and especially low-achieving girls, is concerning from the perspective of equity in health.</p>