The present dissertation brings together research on heritage language acquisition and bilingual education and makes inroads into the limited research on heritage language development in school-aged bilingual children. This study explores how bilingual education, patterns of exposure, and lexical frequency shape adolescent Spanish heritage speakers' knowledge of the subjunctive mood, a complex grammatical structure. The exploration of factors such as these that can account for differences between heritage speakers or within a single speaker's grammar has gained traction in adult research (see Giancaspro et al., 2022), but has yet to be systematically evaluated in children. Moreover, by comparing middle school children enrolled in a dual-language immersion school, in which children learn part of the curriculum in the heritage language, with those in a socioeconomically-matched English-only school, this dissertation takes a novel approach to studying the effects of educational context on heritage language development. The subjunctive mood is a grammatical structure that has proven to emerge in a different (e.g., Montrul, 2009; Montrul & Slabakova, 2011) but highly systematic (e.g., Giancaspro, 2020; Giancaspro et al., 2022; Lustres et al., 2020; Perez-Cortes, 2021a, 2021b) way in Spanish as a heritage language. It is an ideal structure to test in the context of bilingual education precisely because monolingual children continue to acquire it into the school years (e.g., Ahern & Torrens, 2021; Blake, 1983; Dracos & Requena, 2022), and research on Portuguese has shown that quantity of input affects bilingual children's acquisition of this structure into adolescence (Flores et al., 2017). Therefore, if schooling boosts exposure to the heritage language, dual-language immersion schools should facilitate children's acquisition of input-sensitive structures such as the subjunctive that continue to be acquired around or past the start of the school period. The assumption that heritage speakers in dual-language immersion schools have an acquisitional advantage over peers in monolingual schools is widespread but untested (e.g, Lindholm-Leary & Genesee, 2014; Potowski, 2007b). To address these opportunities for investigation, 93 participants carried out an elicited production task and a forced choice task assessing subjunctive mood in two syntactic contexts: volitional clauses following the verb "querer" ("to want"), which are acquired by age five (Blake, 1983; Dracos et al., 2019), and relative clauses, whose acquisition in monolingual populations is contingent upon Theory of Mind (Ahern & Torrens, 2021; Perez-Leroux, 1998) and lasts beyond the beginning of the school period. Participants were divided into five groups: Spanish-dominant comparison adults (n = 18), seventh and eighth grade children in the immersion program (n = 11), seventh and eighth grade children in a monolingual school (n = 23), fifth grade children in the immersion program (n = 19), and fifth grade children in a monolingual school (n = 22). All child participants were heritage speakers of Spanish. This grouping of participants allowed for a "multiple baselines" approach, through which the children were compared to an adult bilingual group that represented their input in Spanish, but also to one another at different age levels and in different contexts of exposure to the heritage language. Results showed that all children used the subjunctive less than the adult comparison group. There were no statistically significant differences at the p < 0.05 level between the children in the immersion and monolingual school groups in either production or selection of the subjunctive mood in volitional clauses. There was a subtle yet significant advantage for the students in the immersion program in the selection of subjunctive mood in relative clauses, the later-acquired context, but this effect did not extend to production. However, all groups -- including the adult baseline -- used less subjunctive in relative clauses than in volitional clauses. Both groups of seventh and eighth grade children used more subjunctive mood in both contexts and across both tasks than the fifth grade groups, suggesting that bilingual children's acquisition of this structure continues into the adolescent years. All children selected the subjunctive mood in both contexts more frequently than they produced it, which aligns with research on adult HS' tendencies (Giancaspro & Sanchez, 2021; Sherkina-Lieber, 2015; Perez-Cortes, 2016). The frequency of verbs and home language exposure did not affect these results, although children's self-reported use of Spanish had a subtle yet statistically significant correlation with use of the subjunctive. These findings show that dual-language immersion does not automatically confer an advantage in bilingual children's acquisition and mastery of complex grammatical structures such as the subjunctive mood. Although children's use of the subjunctive increased with age, the present study does not align with prevalent assumptions in bilingual education research. Therefore, these results lay the foundation for future research on pedagogies for teaching the heritage language in bilingual schools and for subsequent analyses of other areas of bilingual children's inflectional systems. Furthermore, they show that some of the factors that affect adults -- frequency of use and asymmetrical knowledge between production and comprehension -- also affect older school-aged heritage speakers of Spanish. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]