In an effort to provide the best academic opportunities for all students, many high schools create heterogeneous classes by removing existing academic tracks or by offering only 1 level of course. Research has indicated that separating students by ability often exacerbates existing academic inequity; however, mixed ability classes can also create challenges for teachers and students. The purpose for this case study was to determine the effectiveness of mixed ability classes from the perspective of the teachers. This study was designed and analyzed within the theoretical frameworks of democratic education and constructivism. This case study sought to examine the teacher-perceived benefits and challenges of heterogeneous ability-grouped classes as experienced by teachers and students, and to determine teacher instructional choices in these classes. Data collected from initial interviews, observations, and follow up interviews of 5 teachers and at a small private high school were triangulated and coded for recurring themes. Although all students benefited socially from heterogeneous classes, students gained little academically. Teachers felt unable to provide appropriate instruction to meet all their students' needs and had difficulty assessing student progress. Schools with heterogeneous classes should consider offering academic support classes for students and extensive professional development for teachers. This study promotes positive social change by informing parents, educators, and administrators of the effects of heterogeneous classes on student learning so that all parties may work together to promote inclusive classrooms without sacrificing the quality instruction each student deserves. [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.]