The purpose of this study was to investigate whether any statistically significant differences in mathematics and reading academic achievement and academic gains of male and female students taught in single-sex classes existed when compared to male and female students taught in coeducational classes. This study reported findings from mathematics and reading achievement scores and academic gain scores from 850 fourth and fifth grade participants enrolled in seven high poverty public elementary schools during the 2007-2008 school term. The experimental group consisted of 347 students taught in single-sex classes. The control group contained 503 students who were taught in coeducational classrooms within the same schools. Four distinct groups were analyzed within the study: female students taught in single-sex classes, male students taught in single-sex classes, female students taught in coeducational classes and male students taught in coeducational classes. Students were examined in three ways: as fourth and fifth grade students combined, only fourth grade students and only fifth grade students. The findings varied in results. Female students taught in coeducational classes attained higher means in reading achievement levels compared to all other class types. Males taught in single-sex classes showed significantly higher growth in reading and mathematics than all other class types. In some instances there were no differences noted in reading or mathematics achievement scores or academic growth between the groups. While the results of this study have implications for all professionals who work with public school students, special attention has been given to implications for school leaders in high poverty schools where gender achievement gaps are prevalent. [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.]