This study uses administrative data to measure the effect of attending a charter school in Denver, Colorado on the achievement of reclassified English as New Language (ENL) students during their monitor period in the mainstream classroom environment. We utilize school choice data from Denver's Common Enrollment System (CES) to control for each school a family selected entering grade six, a typical time for a structural move, in order to better control of unobserved variable bias. Results indicate charter attendance increases achievement in mathematics and English on state testing. Charter school success in the CES environment provides insights for districts serving large numbers of ENL students.