One of the most wide-ranging reforms in public education in the last decade has been the reorganization of large comprehensive high schools into small schools with roughly 100 students per grade.  We use assignment lotteries embedded in New York City's high school match to estimate the effects of attendance at a new small high school on student achievement. More than 150 unselective small high schools created between 2002 and 2008 have enhanced autonomy, but operate within-district with traditional public school teachers, principals, and collectively-bargained work rules.  Lottery estimates show positive score gains in Mathematics, English, Science, and History, more credit accumulation, and higher graduation rates. Small school attendance causes a substantial increase in college nrollment, with a marked shift to CUNY institutions. Students are also less likely to require remediation in reading and writing when at college. Detailed school surveys indicate that students at small schools are more engaged and closely monitored, despite fewer course offerings and activities.  Teachers report greater feedback, increased safety, and improved collaboration. The results show that school size is an important factor in education production and highlight the potential for within-district reform strategies to substantially improve student achievement.