The U.S. Department of Education is giving more liberties to school districts to offer single-sex schools in order to adequately serve the needs of students. The purpose of this quantitative causal-comparative study was to test the theory of students' performances based on their educational environment by comparing students who received instruction in a single gender classroom in a public middle school compared to students who received instruction in a mixed gender classroom in a public middle school. The achievement test, Georgia Criterion Referenced Competency Test, was used with each site school. There were two years of data collected with the standardized test and was utilized as a pretest and posttest in all subtests areas. The data were compared as whole group, females to females, and males to males. The findings in the study showed that there were significant differences for the whole group in reading, science and social Studies. There were also significant differences in the males in science, and social studies; females showed significant differences in math, reading, science, and social studies. [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.]