Teachers are uniquely placed to comment on the psychosocial functioning of their students. In particular, teacher report of symptoms and functional impairment is crucial in a diagnostic assessment of Attention-deficit Hyperactivity Disorder (ADHD). For adolescents, however, schooling structures and other factors can influence the reliability of teacher reports. Clarity is needed for both clinicians and researchers regarding the interteacher reliability across different domains in the assessment of adolescents. This study investigated interrater reliability of teacher reports of adolescents using data from the 72-month follow-up of the National Institute of Mental Health (NIMH) Collaborative Multisite Multimodal Treatment Study of Children with ADHD (MTA) when participants were 13–15.9 years old. For adolescents with a history of ADHD (MTA; N = 177–210), and a normative comparison group (Local Normative Control Group [LNCG]; N = 100–125), intraclass correlations (ICC) were examined between Math and English teacher reports of ADHD symptoms, externalizing behavior, scholastic competence, and social functioning. Results indicate poor to moderate reliability in the assessment of adolescents with a history of ADHD for core ADHD symptoms, social functioning and scholastic competence, and moderate to good reliability of externalizing behavior. Interteacher reliability was better for the normative comparison group in all domains except social functioning, which was also poor to moderate. Clinicians and researchers should be aware of potential inconsistencies in teacher reports and where possible collect multiple teacher reports to maximize reliability. Further implications for research and clinical practice are explored.