Objective This study examined early predictors of and changes in school‐age academic achievement and class placement in children referred for autism spectrum disorder (ASD) at age 2. Method Of 111 ASD referrals, 74 were diagnosed with ASD at age 18. Regression analyses were performed to identify age 3 predictors of achievement in arithmetic, passage comprehension, word reading, and spelling at ages 9 and 18. Linear Mixed Models were used to examine predictors of academic growth between ages 9 and 18. Results Academic skills varied widely at 9 and 18, but were mostly commensurate with or higher than expected given cognitive levels. However, 22% (age 9) and 32% (age 18) of children with average/above average IQ showed below/low average achievement in at least one academic domain. Children who remained in general education/inclusion classrooms had higher achievement than those who moved to special education classrooms. Stronger cognitive skills at age 3 and 9 predicted better academic achievement and faster academic growth from age 9 to 18. Parent participation in intervention by age 3 predicted better achievement at age 9 and 18. Conclusions Many children with ASD achieve basic academic skills commensurate with or higher than their cognitive ability. However, more rigorous screening for learning difficulties may be important for those with average cognitive skills because a significant minority show relative academic delays. Interventions targeting cognitive skills and parent participation in early treatment may have cascading effects on long‐term academic development.