Background: Attention-deficit/hyperactivity disorder (ADHD) is associated with negative social and occupational outcomes across the life course. However, there has been limited population-based research that quantifies the impact of ADHD on academic achievement and academic trajectories. Aims: To compare academic performance and academic trajectories of students with ADHD with students without a mental disorder. Sample: Data were drawn from Young Minds Matter, a national population-based sample of 6,310 Australian children and adolescents aged 4-17 years. Using linked achievement test data, the academic performance and trajectories of 327 students with ADHD were compared with those of 3,916 students without a mental disorder. Methods: Survey data were combined with scores on national standardized tests for literacy and numeracy over an 8-year period. Results: In Year 3, students with ADHD were on average 1 year behind students with no mental disorder in reading and numeracy, and 9 months behind in writing. In Year 9, the gaps were much larger with students with ADHD on average 2.5 years behind in reading, 3 years behind in numeracy, and 4.5 years behind in writing. Conclusions: Students with ADHD have substantially lower achievement in reading, writing, and numeracy. Writing was the most adversely affected domain. For example, in Year 9 students with ADHD were on average writing at a Year 5 level. Children and adolescents with ADHD need substantial support to manage inattention, impulsivity, and hyperactivity. Skilled remediation in literacy and numeracy is required throughout all school years.