Although existing research suggests that students benefit on a range of outcomes when they enroll in early algebra classes, policy efforts that accelerate algebra enrollment for large numbers of students often have negative effects. Explanations for this apparent contradiction often emphasize the potential role of teacher and peer effects, which could create positive effects for individual students placed into early algebra that would not translate to larger-scale policies. We use detailed data from Oregon that contain information on the teachers and peers to whom students are exposed in order to investigate these explanations. Our regression discontinuity analyses replicate key findings from prior studies, indicating that placement in eighth-grade algebra boosts student achievement in math and English language arts. We then demonstrate that eighth-grade algebra placement positively affects the achievement level of students’ classmates, as well as the years of experience and value added of students’ math teachers. The effects on peer composition that we observe are large enough to plausibly explain the majority of the effects of eighth-grade algebra on student test scores.