Sixteen meta-analyses were conducted to examine relations of typically developing bilingual immigrant-background children's oral language proficiency in their first and second language with the school outcomes of early literacy (k = 41), reading (k = 61), spelling (k = 9), mathematics (k = 9), and academic achievement (k = 9). Moderate to strong within-language relations were found for all school outcomes (0.22 < r < 0.43), and cross-language relations for early literacy and reading (0.12 < r < 0.22). Within-language relations were stronger than cross-language relations (0.14 < d < 0.35). Only 6 out of 96 moderator effects tested were significant. Based on our findings, we propose a task-dependent bidirectional transfer hypothesis: The strength of cross-language transfer depends on the type of language proficiency task and the type of school outcome. Stimulating oral language proficiency in both languages can be a key factor in improving school outcomes of bilingual immigrant background children.