Effortful control refers to the propensity to regulate one's impulses and behaviors, to focus and shift attention easily, and to motivate the self toward a goal when there are competing desires. Although it seems likely that these capacities are relevant to successful functioning in the school context, there has been surprisingly little longitudinal research examining whether youth with poor effortful control are more likely to act out in the classroom, get suspended, and skip school. Conversely, there is even less research on whether youth who exhibit these school behavioral problems are more likely to decline over time in effortful control. We used multimethod data from a longitudinal study of Mexican-origin youth (N = 674), assessed biennially from 5th to 11th grade, to examine the codevelopment of effortful control and school behavioral problems. Bivariate latent growth curve models revealed a negative association between the trajectories of effortful control and school behavioral problems, indicating that steeper decreases in effortful control were related to steeper increases in school behavioral problems. Furthermore, this codevelopmental pattern was bidirectional; cross-lagged regression analyses showed that low effortful control was associated with relative increases in school behavioral problems, and school behavioral problems were associated with relative decreases in effortful control. Gender, nativity status, Mexican cultural values, and school-level antisocial behavior had concurrent associations with effortful control and school behavioral problems, but they did not moderate the codevelopmental pathways. We discuss the theoretical implications for personality development, as well as the practical implications for reducing school behavioral problems during adolescence. (PsycINFO Database Record (c) 2019 APA, all rights reserved).