As the number of K-12 English language learners (ELLs) continues to increase, enhanced education policy is needed to meet their academic needs. Using regression discontinuity in test scores I analyze the optimal English proficiency level and timing of entering and exiting English as a second language (ESL) programs. I find marginal kindergarteners receive small gains from ESL classes. Marginal 2nd to 4th graders receive large benefits in their English test scores (0.25 SD) and GPA that persist over the next 7 years when reclassified from ESL to mainstream English classes. Boys receive the majority of the benefit. Older grades receive no benefits. Achievement gains are available by increasing the inflow and early outflow of ELLs.