Relying on the results of the achievement tests in mathematics, science, native language (Hebrew/Arabic) and English, administered to 1430 5th-grade co-educational classes in Israel, this study examines the between-class variability of the within-class mean score gender differences and its class and school correlates. The four main results of the study are: (1) remarkable between-test stability of the within-class gender gap; (2) considerable variability of the within-class gender gap, in terms of both sign and magnitude, found for each of the four tests: children studying in different classes are exposed to different, sometimes quite opposite, gender differences and this variability is effectively masked by the aggregate-level analyses typically reported in the literature; (3) the lion's share of the variability of the within-class gender gap lies within, rather than between schools; and (4) the relative frequency of within-class gender gaps favouring boys is positively related to school-level characteristics, which qualify as positive indicators of the school's quality and negatively to class size. The within- and between-sector (Arab vs. Jewish) components of this relation are discussed.