With hundreds of primary schools to choose from, young adolescent girls in Nairobi’s urban informal settlements commonly transfer schools. We qualitatively investigate the causes and quantitatively investigate the consequences of such mobility. Key reasons for transferring schools include difficulty in paying fees and anticipated net benefits from a different school. Transferring during lower primary leads to a poorer resourced school, while transferring during upper primary a higher resourced school. Correspondingly, transfers during lower primary are associated with falling behind, while in upper primary with getting ahead, as well as with improved basic reading and math test scores.