It has long been hypothesized that swimming in schools allows fish to save energy. To save energy, fish must extract energy from the wakes of their neighbors, a feat that requires them to stay in certain positions relative to their neighbors and synchronize their tail movements to their neighbors. However, how fish synchronize their swimming movements is still not well understood. Fish can use their flow sensing lateral line system to maintain position in a school. The lateral line system is divided into two branches. The anterior branch, located on the head, encounters largely undisturbed free-stream flow, while the posterior branch, located on the trunk and tail, encounters flow that has been affected strongly by the tail movement. Thus, we hypothesize that the anterior branch may be more important for regulating position within the school, while the posterior branch may be more important for synchronizing tail movements. Our study examines functional differences in the anterior and posterior lateral line in the structure and tail synchronization of fish schools. For one fish in the school, we ablated only the anterior lateral line, only the posterior portion, or all of the lateral line by protecting either the head or the trunk with low melting point agarose during hair cell ablation treatments. Ablating any portion of the lateral line system causes fish to swim in a “box” or parallel swimming formation, different from the control diamond pattern. Ablating only the anterior lateral line did not substantially reduce tail beat synchronization but ablating only the posterior lateral line caused fish to stop synchronizing their tail beats, largely because the tail beat frequency increased dramatically. Most importantly, the posterior lateral line system played a major role in determining tail beat synchrony in schooling fish.