Personality research has mainly relied on self-report measures, more than on behavioral assessments. In the present study, brief behavioral personality tests were developed to measure behavior that is prototypical for the Big Five traits of conscientiousness and agreeableness. A longitudinal study (N = 322; mean age = 12.2 years) examined the predictive value of these newly developed personality tests on changes in academic and social adjustment across the transition to secondary school. Conscientiousness and agreeableness were measured before the transition to secondary school using both the behavioral personality tests and a self-report questionnaire. Academic achievement and social acceptance were measured both pre- and post-transition. The behavioral test of conscientiousness predicted changes in children’s school achievement across the transition to secondary school, above and beyond self-reported conscientiousness. Similarly, the behavioral test of agreeableness predicted change in social acceptance, whereas self-reported agreeableness did not. Behavioral personality tests index unique aspects of children’s behavioral dispositions that are not fully captured by traditional self-report personality questionnaires, and that have predictive power over time.