Using advanced quantitative methods, this article demonstrates that cumulative exposure to the school language is the best language experience predictor of proficiency in that language (as indexed by sentence repetition, lexical semantic, and discourse semantic tasks) in a highly diverse group of 5- to 7-year-old bilingual children in monolingual education. An objective method is proposed to identify the amount of school language experience beyond which bilingual children are likely to perform within the monolingual range, and show that relative passivity in the home language does not translate into better school language proficiency. Socioeconomic status is shown to interact in complex ways with language exposure, such that it is only above a certain level of exposure to the school language that the benefits of a more privileged background have a tangible impact on school language proficiency. To tease apart the effect of environmental predictors from the effect of cognitive factors, memory and cognitive flexibility measures are included as covariates in all analyses.