The SEED schools, which combine a No Excuses charter model with a 5-day-a-week boarding program, are America''s only urban public boarding schools for the poor. We provide the first causal estimate of the impact of attending SEED schools on academic achievement, with the goal of understanding whether changing a student''s environment is an effective strategy to increase achievement among the poor. Using admission lotteries, we show that attending a SEED school increases achievement by 0.211 standard deviation in reading and 0.229 standard deviation in math per year. However, subgroup analyses show that the effects may be driven by female students.