Lottery estimates suggest oversubscribed urban charter schools boost student achievement markedly. But these estimates needn't capture treatment effects for students who haven't applied to charter schools or for students attending charters for which demand is weak. This paper reports estimates of the effects of charter school attendance on middle-schoolers in charter takeovers in New Orleans and Boston. Takeovers are traditional public schools that close and then re-open as charter schools. Students enrolled in schools designated for closure are eligible for "grandfathering" into the new schools; that is, they are guaranteed seats. We use this fact to construct instrumental variables estimates of the effects of passive charter attendance: the grandfathering instrument compares students at schools designated for takeover with students who appear similar at baseline and who were attending similar schools not yet closed, while adjusting for possible violations of the exclusion restriction in such comparisons. Estimates for a large sample of takeover schools in the New Orleans Recovery School District show substantial gains from takeover enrollment. In Boston, where we can compare grandfathering and lottery estimates for a middle school, grandfathered students see achievement gains at least as large as the gains for students assigned seats in lotteries. A non-charter Boston turnaround intervention that had much in common with the charter treatment generates gains as large as those seen for takeovers, but other more modest turnaround interventions produce much smaller effects.