This contribution presents a tool elaborated from a theoretical framework linking epistemological, learning and didactical hypotheses. This framework lead to design teaching sequences from a socio-constructivist perspective, and is based on the role of models in physics or chemistry, and on the role of students' initial knowledge in learning processes. This tool, formatted as a 'grid', is applied to one example in physics (optics, grade 11), and to one example in chemistry (conductivity, grade 11). Both these examples are taken from the important activity our team has developed from several years, in collaboration with upper secondary school science teachers, in order to design teaching sequences and experiment them in real classrooms.