We use admissions lotteries to estimate effects of attendance at Boston's charter high schools on college preparation and enrollment. Charter schools increase pass rates on Massachusetts' high-stakes exit exam, with large effects on the likelihood of qualifying for a state-sponsored scholarship. Charter attendance also boosts SAT scores sharply and increases the likelihood of taking an Advanced Placement (AP) exam, the number of AP exams taken, and AP scores. Charters induce a substantial shift from 2- to 4-year institutions, though the effect on overall college enrollment is modest. Charter effects on college-related outcomes are strongly correlated with charter effects on earlier tests.