Employing a mixed-methods research design, this study examines how a newly designed dual language program in an urban school advances language proficiencies among Spanish-English bilingual 6th graders in relation to content area achievement as measured on NYS standardized tests in English Language Arts and Math. It further investigates how students' emotional (school identification) and behavioral engagement (language learning commitment) relate to bilingual language proficiencies. The results drawn from both quantitative and qualitative data indicate that bilingual students benefitted from attending a dual language program. It contributes positively to students' academic achievement, bilingual school identification and commitment to language learning. With respect to language development, the study found that English proficiency in productive skills was positively correlated with standardized test scores. Spanish proficiency was positively correlated with students' commitment to language learning. Spanish and English languages play different but equally important functions in the dual language program. However, students, face challenges in sustaining learning the two languages in school; we show that students experience a gradual language shift from Spanish to English. The study underscores the potential benefits of a "multilingual proficiency" perspective theory among the students and in preparing bilingual teachers.