<p>Academic underperformance is a common problem of pupils with attention-deficit hyperactivity disorder (ADHD). The current study investigates two longitudinal cohorts of pupils over three years, beginning when they were on average 12 years of age. Cohort 1 consisted of pupils with ADHD and a control group (<i>N</i> = 282) that were assessed from 2012 to 2015. Cohort 2 consisted of pupils with ADHD and a control group (<i>N</i> = 170) that were assessed from 2019 to 2022. Control groups were matched by age in months, gender, and dyslexia leading to a total sample of <i>N</i> = 368. A constant and consistent achievement gap between the two groups could only be found in both mathematics and language grades when ADHD was not comorbid with dyslexia. In the smaller sub-samples of pupils with both ADHD and dyslexia, no significant performance differences could be found due to large differences between individuals. Thus, the co-occurrence of ADHD and dyslexia can obfuscate the academic performance gap that is one of the core problems of ADHD. Moreover, path modeling showed that influence of ADHD on school grades was significant at the beginning but diminished towards the end of secondary education. Extraneous factors such as parent education and migration background as well as pupils’ gender were correlated with school grades especially in the more recent cohort 2, most likely because of fewer pupils were on an academic trajectory. A broad educational policy implication is to support pupils from disadvantaged parental backgrounds.</p>