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The passing of the Education of the Handicapped Act (EHA) of 1970, as well as subsequent education policy, including the Individuals with Disabilities Education Act (2004), have been pivotal to ensuring that both the civil and educational rights of students with disabilities continue to be promoted and protected within educational settings. In order to effectively advocate for and identify students with disabilities, however, IDEA (2004) outlined a special education typology that has been implemented throughout the United States as a method of identifying and categorizing students found eligible for special education services. Much has been debated within the education literature regarding the reliability and external validity of this special education classification system, with many authors cautioning that research supporting either is limited. This study contributed to the literature examining special education disability typologies generally, and high-incidence disability groups specifically, by evaluating a particular criterion for external validity: the suggestion that subcategories within a typology should reflect differential levels of academic achievement, academic growth trajectories, and academic outcomes. Multiple analyses were used, including hierarchical linear modeling, comparison of effect size estimates, analysis of variance, and chi-square. Results of these analyses revealed significant differences in the performance of students with mild intellectual disabilities as compared to students with either emotional disturbance or specific learning disabilities; in addition, this pattern was observed across academic domains. By contrast, results suggested that significant differences in academic outcomes, average achievement levels, and academic trajectories were observed between students with specific learning disabilities and students with emotional disturbance in math specific analyses; however, no significant difference between students in these two high-incidence disability categories were observed across analyses in reading. Thus, although current results support the external validity of the special education typology for students with mild intellectual disabilities, less clear conclusions regarding the external validity of the classification system as it relates to students with specific learning disabilities and emotional disabilities may be drawn. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]