The researcher developed a case study using a qualitative research methodology to describe the functions of a team of math teachers in a professional learning community in a Title 1 low performing high school that led to increased student achievement. The tenth-grade Texas Assessment of Knowledge and Skills (TAKS) mathematics results for the 2011-2012 school year were used to explore the relationship between professional learning communities and academic achievement in a Title 1High School in South Texas that served a large population of economically disadvantaged, at-risk students. This case study attempted to uncover the processes, strategies, and practices used by the math department within the professional learning community that led to improvement and a narrowing of the learning achievement gap by all of the tenth-grade students, including all of the subgroups as identified by the NCLB act, on the mathematics portion of the TAKS test. After a careful review of the data, the researcher coded the interviews and found that many of the characteristics of a professional learning community as defined by DuFour and Eaker (1998) in "Professional learning communities at work: Best practices for enhancing student achievement," were inherent in the processes, strategies, and practices utilized by the math team in the professional learning community during the 2011-2012 school year at Excellence High School, a pseudonym for the research site. This math team that acted in a professional learning community had a positive effect on the teaching and learning at Excellence High School where the majority of students were economically disadvantaged. It transformed a school that had not met Adequate Yearly Progress in mathematics and increased their scores from 69% of the students to 85% of the students meeting the performance standards in the mathematics TAKS test. [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/en-US/products/dissertations/individuals.shtml.]