This paper uses student level data from New York City to study the relationship between a public school losing enrollment to charter school competitors and the academic achievement of students who remain enrolled in it. Geographic measures most often used to study the effect of school choice policies on public school student achievement are not well suited for densely populated urban environments. I adopt a direct approach and measure charter school exposure as the percentage of a public school's students who exited for a charter school at the end of the previous year. Depending on model specification, I find evidence that students in schools losing more students to charter schools either are unaffected by the competitive pressures of the choice option or benefit mildly in both math and English. (Contains 5 tables.)