Despite the prevalence of school tracking, evidence on whether it improves student success is mixed. This paper studies how tracking within high school impacts high-achieving students' short- and longer-term academic outcomes. Our setting is a large and selective Chinese high school, where first-year students are separated into high-achieving and regular classrooms based on their performance on a standardized exam. Classrooms differ in terms of peer ability, teacher quality, class size, as well as level and pace of instruction. Using newly collected administrative data and a regression discontinuity design, we show that high-achieving classrooms improve math test scores by 23 percent of a standard deviation, with effects persisting throughout the three years of high school. Effects on performance in Chinese and English language subjects are more muted. Importantly, we find that high-achieving classrooms substantially raise enrollment in elite universities, as they increase scores on the national college entrance exam--the sole determinant of university admission in China.