Philadelphia offers an extensive array of public high school options for students, including charter schools, traditional schools, schools with selective admissions, and schools at which admission is based on residence (that is, neighborhood catchment zones). A large number of education options might benefit students by enabling them to find schools that are the right fit for them. But a choice-rich education system might also increase student mobility from one school to another during high school. For example, schools might counsel students experiencing academic or behavioral problems to enroll elsewhere, while the extent of school choices might lead some students to continue shopping for schools throughout their high school years. The purpose of this study is to understand the extent of school mobility among Philadelphia public school students--in both charter schools and traditional district schools--during grades 9-12. The study also examines the characteristics of mobile students and the types of schools they most commonly exit and enter, as well as the characteristics of schools with higher rates of student mobility. The study concludes with an examination of the association between student mobility and high school dropout.