The present study examined the atypically wide gender gap in reading performance in Finland. The aim was to provide novel insights via two types of analyses in a longitudinal sample across Grades 1 through 9 (N = 1,512). First, we examined whether girls’ superiority is reading specific or if it can also be found in arithmetic fluency. Second, we examined whether the gender differences in students’ experiences of school, including enjoyment, engagement, task value for literacy, or burnout, could explain the gender difference in reading fluency and Program for International Student Assessment (PISA) Reading performance. The results suggested no general advantage for girls, instead indicating a slight but consistent advantage for boys in arithmetic fluency. Furthermore, the gender effect in reading was significant but at a customary low level, predicting 9% of the reading fluency variance and 5% of the PISA Reading variance. A part of the gender effect on reading ran indirectly via school enjoyment, task value for literacy, and exhaustion, but for the most part, it was not mediated via the students’ experiences of school. However, the gender effect on PISA Reading was fully mediated via reading fluency. Of the PISA Reading variance, 41% was explained by the model based on gender, students’ experiences of school, and reading fluency. The results suggest that gender effect in reading (1) is not as large as often thought but is visible in both reading fluency and in reading comprehension (PISA), (2) is not a reflection of an overall poorer school achievement of boys, and (3) is only partially mediated via students’ experiences of school.