Background/Context: From two decades of research on the grading practices of teachers in secondary schools, researchers discovered that teachers evaluated students on numerous factors that do not validly assess a student's achievement level in a specific content area. These consistent findings suggested that traditional grading practices evolved to meet the variety of educational stakeholder expectations for schools, teachers, and students. Purpose/Objective: The purpose of this study was to examine the role of standards-based grading in a high school reform by assessing the relationships between differing grading approaches and standardized test achievement. Setting: The study examined student performance from 11 high schools operating in a large metropolitan school district. Population/Participants: The sample of students included two cohorts of 1,163 and 1,256 11th grade students who completed an Algebra 2 course and the state standardized test. Intervention/Program: Each of the high schools implemented a locally designed reform known as Project Proficiency. A key component of the reform included utilizing standards-based grading to assess student proficiency of the content. Research Design: This study utilized a non-equivalent control group design and quantitative analyses to compare the association between classroom grades and standardized test scores. Data Collection and Analysis: The data for the study included the students' final grades, standardized test scores, and basic demographic information. Findings/Results: Results indicated that the rate of students earning an A or B in a course and passing the state test approximately doubled when utilizing standards-based grading practices. In addition, results indicated that standards-based grading practices identified more predictive and valid assessment of at-risk students' attainment of subject knowledge. Conclusions/Recommendations: The article demonstrates the benefits of using standards-based grading in reforms attempting to improve the academic performance of secondary schools, but also notes how restriction of grades to mastery of standards will challenge educators' perception of their abilities and students' efforts. The article also notes the methodological limitations of prior grading research and suggests the need for more robust studies assessing grading practices, student achievement, and school performance.