The study aimed to examine the relationship between self-knowledge of trustworthiness and young children's school adjustment. One hundred and seventy-three (84 male and 89 female) children from school years 1 and 2 in the United Kingdom (mean age 6 years 2 months) were tested twice over 1-year. Children's trustworthiness was assessed using: (a) self-report at Time 1 and Time 2; (b) peers' reports at Time 1 and Time 2; and (c) teacher-reports at Time 2. School adjustment was assessed by child-rated school-liking and the Short-Form Teacher Rating Scale of School Adjustment (Short-Form TRSSA). Longitudinal quadratic relationships were found between school adjustment and children's self-knowledge, using peer-reported trustworthiness as a reference: more accurate self-knowledge of trustworthiness predicted increases in school adjustment. Comparable concurrent quadratic relationships were found between teacher-rated school adjustment and children's self-knowledge, using teacher-reported trustworthiness as a reference, at Time 2. The findings support the conclusion that young children's psychosocial adjustment is best accounted for by the realistic self-knowledge model (Colvin & Block, 1994).