This causal-comparative study examined the effects of the collaborative leadership style provided by professional learning communities (PLCs) on the students' Tennessee Comprehensive Assessment Program (TCAP) mean Value-Added achievement scores in language arts and mathematics. The study used twenty-nine selected middle schools in eight different East Tennessee counties to determine whether any statistically significant difference existed in the 2006-2009 TCAP mean Value-Added scores at the thirteen middle schools that use a PLC and the sixteen middle schools that did not use a PLC (i.e., NPLC schools). Scholarly literature was examined to establish what prior research revealed about the correlation of PLCs, student achievement, and mean Value-Added scores. A review of literature showed that PLCs have had a positive effect on student achievement. Therefore, this study's purpose was to examine whether implementing a PLC had a positive effect on 2006-2009 TCAP mean Value-Added scores. The methodology used in this study was an independent samples t test to compare seventh and eighth grade language arts and mathematics TCAP mean Value-Added achievement scores in (a) the PLC school system against (b) the neighboring NPLC school systems' with similar demographics (e.g., ethnicity and economic disadvantage regarding free and reduced-price lunch status). The dependent variable for this study was the TCAP mean Value-Added achievement scores and the independent variable was the type of school students attended, PLC or NPLC. The results showed no significant statistical difference in fifteen of the sixteen contexts addressed; however, for the NPLC in 2006 for eighth grade mathematics one statistically significant result for the NPLC schools. [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.]