The continued failure of a large, suburban, Title I high school to meet adequate yearly performance indicators as defined in the No Child Left Behind Act is a concern for school personnel, district officials, and the school community. The purpose of this quantitative study was to determine whether 9th grade teachers working collaboratively, within a Vygotskian framework, following the processes of a professional learning community (PLC) would impact student academic achievement on the Mathematics I End of Course Test (EOCT). An ex post facto design was used to conduct this nonexperimental study which involved the analysis of the student performance scores from the EOCT of the subject school and another high school in the district with a similar population. Descriptive statistics, t tests, and chi-square tests were used to determine if there were statistically significant differences between the scores of the 2 groups as a result of implementing the PLC. The findings of the study indicated a statistically significant difference in the economically disadvantaged groups of students and identified limited evidence of effectiveness for the overall population. The research findings informed the creation of a plan to enhance professional development policies and procedures to implement and sustain PLCs. Implementing the plan throughout the school and in similar schools in the school district may contribute to increased student academic achievement and teacher development. These outcomes may reduce the high school drop-out rate and promote an increase of student matriculation into tertiary education after graduating from high school, which would be a significant social benefit to the community. [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.]