Previous research demonstrates that collective efficacy positively predicts students' academic achievement (e.g., Bandura, 1993; Goddard et al., 2000). However, unaddressed by the current literature is whether collective efficacy also works to reduce inequity by closing achievement gaps. To learn about the operation of collective efficacy, we designed a mixed-methods study, situated in the elementary and middle schools of one large urban district in Texas. Hierarchal linear modeling was employed to model the degree to which collective efficacy explained differences among schools in student mathematics achievement and the Black-White achievement gap. We also drew upon focus group data collected at 6 schools. We found that collective efficacy was associated with an increase in mathematics achievement as well a 50% reduction in the academic disadvantage experienced by Black students. Focus groups revealed the importance of school principals in supporting teacher collaboration and peer observation as well as a sustained focus on instructional improvement.