Background: Students with attention/deficit hyperactivity disorder (ADHD) continue to languish behind their peers with regard to academic achievement and education attainment. School engagement is potentially modifiable, and targeting engagement may be a means to improve education outcomes. Aims: To investigate school engagement for students with ADHD during the crucial high school transition period and to identify factors associated with low school engagement. Sample: Participants are adolescents (12-15 years) in the first and third year of high school with diagnosed ADHD (n = 130). Participants were recruited from 21 paediatric practices. Methods: Cross-sectional study assessing school engagement. Data were collected through direct assessment and child, parent, and teacher surveys. School engagement is measured as student attitudes to school (cognitive and emotional) and suspension rates (behavioural). Multivariable regression analyses examined student, family, and school factors affecting engagement. Results: In comparison with state data, students with ADHD in the first year of high school were less motivated (p < 0.01) and less connected to peers (p < 0.01). Overall, there was no discordance in third year attitudes. There were high rates of suspension in both years in comparison to state-wide suspensions (21% vs. 6%, p < 0.01). Explanatory factors for poor attitudes include adolescent depression, poor adolescent supervision, and devaluing education. Conduct problems and increased hyperactivity were related to increased likelihood of being suspended, whilst higher cognitive ability, family socio-economic status, and independent schools reduced risk. Conclusions: Potentially modifiable individual and family factors including adolescent depression, behavioural problems, education values, and family supervision could be targeted to better manage the high school transition for students with ADHD.