The transition from middle to high school can be especially challenging for students with attention-deficit/hyperactivity disorder (ADHD). Also, students with disabilities experience greater barriers when considering careers in science, technology, engineering, and math (STEM) fields. Academic interest and social support may play a critical role in self-regulatory processes and school success in this period. The current study investigated the roles of school-level support, parent support, teacher emotional support, and teacher academic support on math/science interest development and overall high school math/science high school grade point average (GPA) among about 1,730 ninth grade students with ADHD entering high school (70% male). The average age of students was 14.55 (SD = 0.69). A national dataset from the High School Longitudinal Study of 2009 (HSLS:09) was analyzed using structural equation modeling (SEM). It was hypothesized that all types of social support would positively predict math/science interest development and achievement. It was also hypothesized that ninth and 11th grade math/science interest would mediate the relationship between social supports and achievement. Students with ADHD demonstrated unique motivational and achievement patterns. School-level support did not predict any outcome variables, and parent support negatively predicted science interest. Perceived teacher emotional support positively predicted ninth grade interest across domains. Teacher support for deep cognitive engagement positively predicted math GPA, but teacher support for surface cognitive engagement negatively predicted math/science interest. Surprisingly, academic interest emerged as a negative predictor of GPA in both domains. Mediation effects were observed, but potential suppression effects complicated the interpretation of results. The findings highlight the importance of providing developmentally appropriate and individualized supports for increasing interest and achievement in high school students with ADHD. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]