Purpose: The purpose of this study was to determine the degree of importance that DuFour's nine characteristics of highly effective schools have on closing the academic achievement gap on the California High School Exit Exam, as perceived by high school principals. The study also examined the strategies believed to be most important in developing and sustaining highly effective PLC schools. Methodology: The subjects in the study were 16 high school principals from throughout Los Angeles, Riverside, Orange, and Sand Diego counties. Principals responded to two research instruments: (a) a 29-item questionnaire titled: Characteristics of PLC Schools Questionnaire, and (b) an interview utilizing 10 structured questions that addressed the strategies schools utilized to develop and sustain an effective PLC culture. Findings: Examination of quantitative and qualitative data revealed that principals at PLC schools who closed the achievement gap agreed with DuFour's nine characteristics of highly effective PLC schools to a higher degree than principals of PLC schools that did not close the gap. Principals who closed the gap believed that fundamental to developing a sustainable PLC culture involved: guiding staff through an understanding of the PLC process, providing time for collaboration, analyzing data with discussions, teacher curricular knowledge, shared decision-making, developing common formative assessments, and effective staff development. Conclusions: The study revealed that PLC high schools that closed the gap have more developed PLCs than schools not closing the gap. In addition, this study showed differences with the potential for great impact on schools. Recommendations: Further research is advised: one recommendation would be to develop a similar study using middle school PLC principals. Research coming from that study could be used to compare the findings from this study. Another recommendation would be to conduct a case study of a school district to measure the differences between each school based on DuFour's nine characteristics (questionnaire). This could provide excellent information to district and school staff in determining the effectiveness of their PLC. [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.]