Education in the United States has a foundational public purpose: to prepare students for effective citizenship. The idea that an educated and engaged citizenry is essential to the health of a democracy motivated the creation of government-run "common schools" in the early decades of the nation and remains an important value in modern times. To determine how effective a public charter school might be at encouraging civic engagement, the authors studied the voter-registration and election-participation rates of former students of a charter network dedicated to encouraging such behavior: Democracy Prep, which has as its mission "to educate responsible citizens for success in the college of their choice and a life of active citizenship." The authors find that Democracy Prep has large positive effects on civic participation, increasing its students' voter-registration rates by about 16 percentage points and their voting rates by about 12 percentage points. Given the low registration and voting rates of young adults nationally, these are substantial impacts. And they provide new evidence that an education focused on preparing students for citizenship can, in fact, boost civic participation in adulthood--even when the school in question is not operated by democratically elected officials.