Psychological factors such as grit and self-efficacy have been heralded as powerful predictors of performance. Their joint contribution to the prediction of early adolescents' school success has not been fully investigated, however. The purpose of this study was to examine U.S. elementary and middle school students' (N = 2,430) grit (assessed as perseverance of effort) and self-efficacy, and their predictive relationship with achievement and teacher-rated motivation and competence in reading and math across one school year. Scalar invariance was found for grit and self-efficacy measures across school level, gender, and SES. Older students and students from lower SES reported significantly lower grit and self-efficacy. Girls reported higher grit and reading self-efficacy. Grit was correlated positively with self-efficacy (0.37 = r = 0.66), modestly with teacher ratings in reading and math (0.14 = r = 0.25), and weakly or uncorrelated with achievement (0.03 = r = 0.13). Self-efficacy was positively related to all outcomes (0.21 = r = 0.36). SEM indicated that subject-specific self-efficacy was positively related, and grit weakly or unrelated, to reading and math achievement, controlling for grade level, gender, SES, and prior achievement. An examination of competing mediation models revealed that self-efficacy partially or fully mediated the relationship between grit and school outcomes. Conversely, little evidence supported grit as a mediator of self-efficacy's relationship to outcomes. Time-lagged models across one school year confirmed these conclusions. Findings imply that, to improve student performance, teachers should target students' self-efficacy rather than grit.