The purpose of this qualitative case study was to explore how teacher perceptions describe the effectiveness and the barriers to implementing Universal Design Learning tools in a proficiency-based system. The problem was that students with disabilities experience achievement gaps in the areas of reading, writing, and mathematics in some classes where instructional practices may include universal design principles. This case study was guided by Vygotsky's theory Zone of Proximal Development and the Universal Design Framework. Vygotsky's theory directly addresses a core tenet of proficiency-based education, which is meeting the individual students at their independent level and moving them forward in the progression of academic skills toward proficiency. The questions the study addressed were: How do teacher perceptions of the effectiveness and barriers of universal design implementation impact the achievement gap in the areas of reading, writing, and mathematics? This study used a sample of 31 participants from 10 different schools in schools in the district. The 31 participants completed a voluntary anonymous online survey and optional online interview questions. The thematic analysis provided relative themes from the online survey and interview questions. The results of the data analysis revealed perceptions of effective implementation included instructional practices, professional development, and positive relationships among teachers and students. Further analysis revealed perceptions of barriers to implementation, the presence of student supports, access to professional development, and flexibility of teachers and administrators. This qualitative case study indicated what teachers perceive as effective in implementing UDL tools as well as the perceived barriers for consideration when addressing the academic progress gap for students with disabilities in the proficiency-based system. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]