This paper presents aspects of a longitudinal study assessing integrative bilingual learning based on Content and Language Integrated Learning (CLIL), implemented in three Swiss primary schools. From Grades 1 to 6, three classes of German-speaking pupils were taught 50% of the curriculum, notably mathematics, in Italian or in Romansh as a second language (L2). The longitudinal study concerns the assessment of L2 oral production, oral and written comprehension, as well as mathematics. After a presentation of the bilingual teaching concept we discuss the role of interaction for L2 acquisition, notably the importance of L1/L2 repair sequences to achieve focus on form (Doughty & Williams, 1998). Through the longitudinal analysis of the repair sequences in oral narrative tests, we observe the decreasing use of L1 as a strategy to fill in L2 missing words. We then draw the developmental profile of clause linkage (Berman & Slobin, 1994), giving insight on the development of verbs and of morphological elements in L2. Finally, the paper presents an analysis of the pupils' results in standardised mathematics tests and their qualitative link to oral and written comprehension tests.