At Haverford High School, a suburban high school in Haverford, Pennsylvania, a mathematical achievement gap exists for economically disadvantaged, special education, and Black students. Researchers have found that students in lower tracked classes often receive less rigorous tasks, tasks with lower cognitive demand, and fewer tasks that focus on conceptual learning (Oakes, 1992). Additionally, researchers posit that these demographics often benefit from a rigorous curriculum (Steiner, 2017) with conceptual learning (Montague, 1992) and student mathematical tasks with high cognitive demand (Silver & Stein, 1996). Haverford High School has historically adopted high-quality instructional materials that have a selection of tasks at different levels of cognitive demand which addressed both conceptual and procedural learning. In order to better understand why economically disadvantaged, special education, and Black students were achieving at lower rates than their counterparts, I decided to investigate Haverford High School teachers' current practices. To that end, I performed the following research activities: collected and analyzed achievement data and examined the tracking practices; designed a survey to identify any gaps between the mathematics teachers' beliefs and practices related to the cognitive demand of mathematical tasks; hosted a focus group of a subset of those teachers to explore their beliefs and practices in more depth; and, studied the repository plans from those teachers to see whether or not they were modifying the written curriculum the school adopted. The survey and focus group revealed that the teachers believe in authentically implementing the curriculum. However, results from the analysis on the teacher repository plans uncovered that the teachers are primarily presenting students with procedural mathematical tasks with low cognitive demand. I created several artifacts aimed at improving the situation at Haverford High School. Since the department decided to adopt a new curriculum, I created an online professional development program to help prepare for authentic implementation in the new school year. I also presented the results from the survey, focus group, and repository plan analysis to the Haverford High School teachers and administrators to assist in influencing positive changes in classroom instruction and student learning. Further, I worked with the administrators and teachers to draft a vision statement in order to make a common goal for the mathematics classrooms. Finally, in order for there to be lasting change, I wrote and submitted a grant proposal to the National Council of Teachers of Mathematics to fund Japanese Lesson Study. If the proposal is accepted, it would provide sustained professional development that incorporated researcher-supported elements. Due to the coronavirus pandemic of 2020, my project was focused on studying teacher self-reported beliefs and behaviors, teacher knowledge, teacher planning practices and creating a vision statement with my department, designing a professional development platform, and applying for grant funding for future professional development. Therefore, future projects should investigate the impact of effective professional development on classroom instruction. If Haverford High School is able to follow its vision and continue to incorporate researcher-supported elements into their classroom instruction, there is an opportunity to improve the achievement gap for the economically disadvantaged, special education, and Black students. I make several suggestions in the conclusion of this Education Leadership Portfolio, based on my reading of the research together with the results of this study, to address the mathematical achievement gap of typically underrepresented groups of students at Haverford High School. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]