In 2009 the Massachusetts Teachers Association (MTA) published "Charter School Success or Selective Out-Migration of Low-Achievers? The Effects of Enrollment Management on Student Achievement." The report profiles Boston's high-performing charter high schools and middle schools and calculates attrition rates for individual schools examining the number of students who enrolled in the entry grade at each charter and the number of students who completed the curriculum, or graduated. The report contends that, in 2009, nearly half the students who enrolled in Boston charters did not graduate from them. This paper uses publicly available Department of Elementary and Secondary Education data to explore student attrition and other forms of student movement, such as dropouts, within district and charter schools. It is not a direct response to the now dated MTA report, but it does explore the validity of the claim that Massachusetts charter public schools have higher attrition than their district counterparts because these schools select out weaker students in an effort to produce higher test scores. This report focuses mainly on Boston charter and district schools because Boston has the Commonwealth's largest concentration of high-performing charter schools in the state. Boston also has the highest demand for charters, with a charter school waitlist of over 10,000 students. The report concludes with recommendations for policymakers and others to better understand and communicate the impacts of student mobility on schools, of which attrition is but one facet.