The aim of the study was to understand the relationship between shyness in school-age children and variables such as teacher perception of school achievement, fluid intelligence and self-efficacy in various areas of functioning. The participants were 466 Polish primary school children (boys n = 239, girls n = 227; M age = 9.55 years, SD =0.9). Shyness was negatively associated with school achievement as reported by teachers, fluid intelligence, and self-efficacy. Results from mediation analyses demonstrated that self-efficacy in learning mediated the relationships between shyness and school achievement and fluid intelligence. In addition the relationship between shyness and school achievement was also mediated by the self-efficacy with respect to self-control in school and social situations. The results indicate that shy children's relatively low school achievement and low scores in various standardised tests may be the result of personality differences, e.g. in self-efficacy. Future studies are needed to explore the relationship between shyness and self-efficacy.