We study personnel flexibility in charter schools by exploring how teacher retention varies with teacher and school quality in Massachusetts. Charters are more likely to lose their highest and lowest value-added teachers. Low performers tend to exit public education, while high performers tend to switch to traditional public schools. To rationalize these findings, we propose a model in which educators with high fixed-costs use charter schools to explore teaching careers before obtaining licenses required for higher paying public sector jobs. The model suggests charter schools create positive externalities for traditional public schools by increasing the average quality of available teachers.