Learners today arrive at school with increasingly varied home language repertoires. At the same time, the educational institutions they rely upon for success have historically been constructed around monolingual norms and practices, with the consequence that educational achievement is contingent on the degree to which multilingual and multidialectal students assimilate to the monolingual norm. But does successful learning rely only on proficiency in the language of instruction? How do multilingual and multidialectal language skills support learning in a developing school language? Prior education research on crosslinguistic influence in language and reading comprehension has largely focused on word-level vocabulary, decoding, and morphosyntax. However, understanding school discourse requires mastering not only a rich lexicon but also complex linguistic structures beyond the word level. This dissertation comprises three multimodal studies exploring how crosslinguistic resources beyond the word level support comprehension in a second language or dialect. Studies 1 and 2 examine crosslinguistic sentence integration with Spanish-English bilingual adolescents in grades 6- 8. The first study (n=38) used functional magnetic resonance imaging (fMRI) to investigate functional connectivity of the core language network and nodes associated with syntactic processing as adolescents watched an L2 English science video lesson with varying degrees of sentence complexity. Connectivity in brain regions implicated in syntactic processing was associated with L1 Spanish sentence integration skills and reflected the syntactic complexity of the video discourse. Using a sample expanded from Study 1, the second study (n=59) employed behavioral and eye-tracking measures to examine L2 English expository reading, finding that L1 Spanish sentence integration skills facilitated L2 English reading efficiency and comprehension and moderated the difficulty that syntactically complex L2 texts posed for comprehension. Study 3 (n=42) used electroencephalography (EEG) to focus more specifically on the timecourse of sentence processing in Spanish-English bilingual and Caribbean English bidialectal, young adult university students who were highly proficient speakers of English. Results from this study revealed distinct patterns of sentence processing that differed both across groups and from expected monolingual patterns reported in the literature. Taken together, findings from the three studies illustrate that diverse linguistic experience gives rise to heterogeneous behavioral and neural patterns in processing school discourse, even when controlling for lexical proficiency. Further, the ability to integrate words into longer and more complex sentence structures may be a crosslinguistic resource supporting second language comprehension, both in oral academic lessons and in reading school texts. Findings from this dissertation support a strength-based account of language diversity in schools that complements and moderates monolingual normative comparisons.