This paper examines the effects of armed conflicts between drug gangs in Rio de Janeiro's favelas on student achievement. We explore variation in violence that occurs across time and space when gangs battle over territories. Within-school estimates indicate that students from schools exposed to violence score less in math exams. The effect of violence increases with conflict intensity, duration, and proximity to exam dates; and decreases with the distance between the school and the conflict location. Finally, we find that school supply is an important mechanism driving the achievement results; armed conflicts are significantly associated with higher teacher absenteeism, principal turnover, and temporary school closings.