This study examined the longitudinal relationship between friendship quality (positive and negative) and school achievement among 228 school-age children (51% girls, M = 8.09, SD = 0.41). A three-wave cross-lagged analysis was used to determine the direction of influence between these domains across school years. Findings revealed that: (a) school achievement in second grade decreased negative friendship quality in third grade; (b) school achievement in second grade predicted positive friendship quality in third grade; (c) positive friendship quality in third grade predicted school achievement in fourth grade. The protective role of school achievement against peer relationship impairment and the reciprocal relationship between positive friendship quality and school achievement were underlined. Implications for educational practices for Italian primary schools are discussed.