This research looks at how the diagnosis and systematic development of Cognitive Academic Language Proficiency (CALP) through cross-curricular instruction might foster critical thinking in lower secondary school. The study arose from a common pedagogical concern identified through collaborative teaching: seventh-grade students consistently struggled to produce academically appropriate oral and written responses across subjects, instead relying on colloquial language and descriptive or minimal responses, even when higher-order reasoning was required. This issue was noticeable in English, Russian, History, and Biology lessons, especially in assignments that required explanation, comparison, analysis, and evaluation.
The research question that guides the study is:
How does cross-curricular instruction aimed at developing Cognitive Academic Language Proficiency (CALP) affect students' critical thinking and capacity to create academically suitable oral and written replies across subjects?
This topic is investigated using the following sub-questions:
1. How does focused cross-curricular instruction affect students' use of academic language in both oral and writing tasks?
2. How does the development of CALP affect changes in students' critical thinking performance across subjects?
3.How do students use academic language and reasoning skills across English, Russian, History, and Biology?
The theoretical framework brings together three research lines that complement one another. First, the research is based on Cummins' (2000, 2008) difference between Basic Interpersonal Communication Skills (BICS) and Cognitive Academic Language Proficiency (CALP), which views academic language as cognitively demanding, decontextualized, and necessary for disciplinary learning. This concept offers a perspective on why children who look linguistically competent may struggle with academically demanding activities, especially in content topics.
Second, the research is based on sociocultural theory, which regards language as the major mediational instrument for cognitive development (Vygotsky, 1978; Mercer, 2000). Critical thinking, according to this viewpoint, is formed through socially mediated academic discourse rather than as an internal capacity. 
Third, the study draws on knowledge on critical thinking and language throughout the curriculum. Critical thinking is defined as a collection of transferable behaviors such as analysis, assessment, synthesis, and evidence-based reasoning (Facione, 1990; Kuhn, 1999). Research has repeatedly revealed that these practices are heavily reliant on language and cannot be properly developed without careful attention to academic discourse. 

The project uses a lesson study method, collaborating with instructors of English, Russian, History, and Biology to develop, execute, monitor, and revise classes that emphasize academic language and critical thinking. The same sample of students was watched across topics, allowing for detailed examination of how academic language usage and critical thinking evolved over time and transitioned across disciplinary settings. This design is consistent with European and worldwide goals, highlighting teacher cooperation, reflective practice, and classroom-based research.

The research uses a mixed-methods research design inside a lesson study framework. This approach was chosen to capture quantitative improvements in students' academic language usage and critical thinking, as well as the instructional procedures that enabled these changes. The study was carried out with a single class of seventh-grade students (ages 12-13) in four subjects: English, Russian, History and Biology. Lesson study served as the main methodology. A group of teachers worked together to construct a series of lessons centered on the diagnosis and development of Cognitive Academic Language Proficiency (CALP) and critical thinking. The same set of students was observed in many topics, allowing for a systematic comparison of how academic language and reasoning abilities were used and transmitted in various disciplinary contexts. Each lesson study cycle included collaborative preparation, classroom observations that focused on student language usage and reasoning, and post-lesson reflection. Data gathering used multiple sources to guarantee triangulation. 1. Pre-test and post-test were used to measure students’ CALP and critical thinking. Performance-based exercises asked students to respond orally and in writing to open-ended questions, comparison assignments, and analytical prompts that were similar across subjects. The same assessment rubrics were used by all teachers to evaluate students’ responses in pre- and post-tests. 2. A systematic classroom observations helped to collect qualitative data. Throughout the intervention, we gathered samples of student work from each topic to track developing trends in language and thinking. Reflective notes from lesson study conversations helped teachers make informed instructional decisions and assess cross-curricular alignment feasibility. 3. To find differences between pre- and post-intervention measures, quantitative data were analyzed using descriptive statistics and paired comparisons. Thematic analyses of qualitative data were conducted, with a focus on recurrent patterns in students' academic speech and critical thinking across disciplines. Informed permission, anonymization of student data, and the use of research tools consistent with ordinary classroom practice were among the ethical issues.

The key findings of the study are: • Students shifted from informal, descriptive comments to more organized, coherent, and analytical oral and written answers, including suitable academic vocabulary, logical connections, and evidence-based reasoning. • Students demonstrated development of critical thinking by increasingly identified causal links, supported arguments with evidence, and displayed higher mastery of academic styles common across disciplines. • The data analysis shows that students with medium or lower baseline CALP had the most significant quantitative increases, notably in response coherence and completeness. Students with better starting proficiency displayed qualitative gains, such as improved argumentation clarity, more flexibility in adapting academic language to varied disciplinary settings, and increased metalinguistic awareness. • The study provides empirical evidence that incorporating CALP-focused instruction into cross-curricular teaching practices can improve critical thinking and help students produce academically appropriate discourse. Moreover, it provides a replicable model for integrating CALP-focused tasks to develop critical thinking in lower secondary classrooms.

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