The Influence of Content and Language Integrated Learning Implementation Quality on Content Achievement and Language Development in Spanish Bilingual Secondary Schools

Abstract

Content and Language Integrated Learning (CLIL) has become a widely adopted approach in bilingual education, aiming to promote simultaneous development of subject knowledge and second language proficiency. Despite extensive research supporting its benefits, relatively little attention has been given to how variations in implementation quality influence educational outcomes. This study investigates the relationship between CLIL implementation quality and content-subject achievement in bilingual secondary school programmes in Spain. Guided by Coyle et al.'s (2010) 4Cs framework, Mehisto et al.'s (2008) CLIL quality indicators, and Dalton-Puffer's (2011) synthesis of CLIL research, the study employed a mixed-methods design involving 24 CLIL teachers from 12 secondary schools and 480 students enrolled in science, history, and geography courses taught in English. Implementation quality was evaluated through structured classroom observations using a validated CLIL Quality Rubric (CQR), which measured instructional planning, integration of content and language objectives, learner engagement, scaffolding, and classroom interaction. Student achievement was assessed using Anglicized standardized content tests and compared with matched non-CLIL classes following the same curriculum. Quantitative findings revealed that high-quality CLIL implementation was a strong predictor of superior content achievement, exceeding non-CLIL benchmarks (r = .62, p < .001), whereas low-quality implementation was associated with achievement levels below those of comparison groups. In contrast, improvements in English language proficiency were observed across all CLIL classrooms (d = 0.54), irrespective of implementation quality, suggesting that language exposure alone contributes to linguistic development. Qualitative evidence from classroom observations and teacher interviews further highlighted the importance of effective scaffolding, balanced integration of language and content objectives, and interactive pedagogy in maximizing learning outcomes. The findings demonstrate that implementation quality is a critical yet frequently overlooked determinant of CLIL effectiveness. The study underscores the need for rigorous teacher preparation, continuous professional development, and quality assurance mechanisms to ensure successful CLIL implementation and optimize both academic achievement and language learning in bilingual education contexts.