Does lower teacher quality contribute to short-term and long-term outcomes for disadvantaged students? We leverage transfers of elementary teachers across schools in North Carolina to measure differences in teachers' effects on contemporaneous and future test scores according to students' socio-economic characteristics. We quantify the importance of these differences to account for the observed test score gaps between disadvantaged and advantaged students. Variation in teacher quality accounts for 3 percent of the total variation in contemporaneous test scores. We also find that teacher quality accounts for similar proportions when we consider variability in test scores taken two and three years after. Our estimates are robust to bias-correction methods that account for limited mobility bias.