The significant growth of charter schools in the United States has brought praise for the excellent results achieved by some schools as well as criticism that charter schools may not be serving the most disadvantaged students. Critics of charter schools, in New York City and elsewhere, commonly assert that charters' (often) strong academic performance derives primarily from the type of student educated, rather than the quality of schooling provided. In particular, many charter school opponents argue that charters systematically "push out" low-performing, or otherwise difficult-to-educate, students in order to boost aggregate test scores. This paper uses longitudinal New York City student-level enrollment data to assess such claims. Key findings include: (1) Low-performing students are more mobile, regardless of where they are enrolled. In New York City charters as well as traditional public schools, low-performing students are more likely to change schools than their higher-performing peers; (2) Low-performing students are not more likely to exit New York City charters than traditional public schools; and (3) To the extent that higher attrition rates for low-performing New York City students offer cause for concern, they are no less a problem for the city's traditional public schools than they are for its charters.