This study examined the impact of NISL's [National Institute for School Leadership's] Executive Development Program for principals on student achievement in Pennsylvania schools between 2006-2009. Roughly half of the NISL-trained principals started the program during the 2007 school year and completed it in the 2008 school year, whereas the other half started during the 2008 school year and completed it in the 2009 school year. Schools served by principals participating in the Executive Development Program were individually matched to comparison schools with similar school performance and demographic profiles in 2006, which served as the baseline year. For elementary schools, it was possible to make individual school matches within the same school district for 36 school pairs. An additional 32 elementary schools were included in an out-of-district matched comparison sample. For all middle and high schools, it was necessary to match outside the school district. The percentages of students achieving proficient or above in mathematics and reading or English/Language Arts (ELA) across all grade levels were used to create aggregate school performance indices for each year 2006-2009. Repeated-measures analyses were performed to determine whether there were differences in school performance trends between schools served by NISL-trained principals and matched comparison schools. At the elementary level, statistically significant differences in school performance trends were observed between the matched within-district NISL and comparison schools in both mathematics and reading/ELA. For both subjects, NISL schools had statistically significantly higher rates of improvement in school performance than did comparison schools. Further, a significant quadratic effect was observed for mathematics, indicating an acceleration in the rate of improvement for NISL schools. In terms of unadjusted results, NISL schools gained 3.9% versus 0.7% for comparison sites in mathematics, and 4.1% versus 3.7% in reading/ELA. For the matched out-of-district elementary schools, significant trends favoring NISL over comparison schools were also indicated in both mathematics and reading/ELA. Significant quadratic effects in mathematics further revealed acceleration in the rate of growth over time for NISL schools. As with the elementary school results, statistically significant positive effects of NISL status were observed for both mathematics and reading in the middle school sample. Unadjusted results showed that NISL students improved the percentage of students achieving proficiency in mathematics by 5% from 2006 to 2009, compared to a 1% improvement in comparison schools. In reading/ELA, NISL middle schools improved the percentage of students achieving proficiency by 3% versus 1% in comparison schools. As with elementary schools, a significant quadratic effect on mathematics school performance trends revealed that the rate of improvement was accelerating over time in NISL schools. Two appendixes present: (1) Supplementary Tables; and (2) Evaluation Rubric for Selecting Program Participants. (Contains 9 tables and 17 figures.)