Two-way dual language (DL) classrooms enroll students of two different language backgrounds and teach curriculum in both languages. I estimate the effect of attending a dual language school on student achievement using school choice lotteries from Charlotte Mecklenburg School District, finding local average treatment effects of 0.04 and 0.05 standard deviations per year in math and reading, respectively. Attending a DL school increased test scores for English learners (ELs) and for non-ELs by a similar magnitude. The positive effects of winning the lottery to attend a DL school are substantially larger than the average effect of assignment to other magnet schools, and several peer and school characteristics are ruled out as explanations for the increased test scores. There is no statistically significant evidence that attending a dual language school changed the probability of having EL status throughout elementary school.