In the fall of 2011, the state of Nebraska experienced a watershed moment in public education: results of the first standardized mathematics achievement test administered to all students in grades 3-8 and 11 the previous spring (2011) were reported to the public. One state senator referred to some of the results as "shocking." Only four of the seven grade levels tested in the research district of this study scored even slightly above the state's determined cut score for proficiency. Also in the fall of 2011, new K-12 mathematics curriculum was being implemented in the research district. The purpose of this study was to explore the effects of that new mathematics curriculum on Nebraska State Accountability-Mathematics (NeSA-M) student cohort scores from 2011 to 2012. Thus, this study compared exact student matches of mean scale scores on the 2011 NeSA-M after the implementation of traditional mathematics curriculum to their mean scale scores on the 2012 NeSA-M after the implementation of new district mathematics curriculum. There were statistically significant differences of student scores in all four student cohorts studied from 2011 to 2012: Cohort 1 (n = 595), Cohort 2 (n = 572), Cohort 3 (n= 623), and Cohort 4 (n = 625). There were medium effect size differences in Cohorts 1and 3, and large effect size difference in Cohorts 2 and 4. This study suggests that replicating the formula for what comprises district curriculum in other disciplines may be prudent. The study suggests further research on the effects of professional development on the teaching and learning of mathematics. The study also suggests consideration of district policy on mathematics courses, mathematics requirements, and the elimination of low-level tracked mathematics courses. [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.]