Teaching children requires effort, and some children naturally require more effort than others. In this study, we tested whether teacher effort devoted to individual children varies as a function of each child's personal characteristics. In a nationwide longitudinal study of 1,102 pairs of twins followed for 7 years, between the ages of 5 and 12 years, we asked teachers about the effort they invested in each child in our study. We found that teacher effort was a function of heritable child characteristics, that a child's challenging behavior assessed at 5 years of age predicted teacher effort toward the same child at 12 years of age, and that challenging child behavior and teacher effort share a common etiology with respect to children's genes. We found that child effects accounted for a significant proportion of variance in teacher effort, but also observed variation in effort exerted by teachers that could not be attributed to children's behavior. Treating children who exhibit challenging behavior and enhancing teachers' skills in managing such behavior could increase the time and energy teachers have to deliver their curriculum in class.