This sequential, explanatory, mixed methods study investigated the relationship between teachers' implementation of the National Council of Teachers of Mathematics (NCTM) and student achievement on Exemplars, a performance-based assessment, in a suburban elementary school in the southeastern United States. The problem addressed was the need to measure implementation of NCTM standards and the influence standards implementation has on student achievement on a performance assessment. Georgia implemented a new curriculum, Georgia Performance Standards (GPS), in kindergarten through second grade in 2006–2007. The GPS are designed to achieve a balance of concepts, skills, and problem solving, and the rigor and depth of knowledge is far greater than the previous curriculum. These standards make heavy demands and require professional development of mathematics teachers. Quantitative data from a self-report survey on teachers' commitment to NCTM standards was correlated with Exemplars. The qualitative component of teacher observation provided corroboration of the self-report survey and added reliability to the survey. The Pearson correlation approached significance, and the ANOVA revealed significance. The post hoc comparisons using the Scheffé revealed a significant difference between medium and high implementation groups. The observations were congruent with the self-report survey. Teacher self-efficacy emerged as a contributing factor to the significant difference found between implementation levels. This research has potential to guide best teaching practices, to support development of communities of learners, to give direction for designing professional development, and to provide recognition for the critical factor of self-efficacy in teaching. The potential for social changes in mathematics may guide improvement in student achievement and develop proficient mathematicians in the elementary setting.