Current international documents claim that schools should enhance mathematical modelling competencies in students, as part of an instructional approach that can be considered suitable for the 21st century learners and problem solvers. The objective of this paper is to identify high-school students’ initial conceptions and strategies in mathematical modelling that can be taken into consideration when teachers work out educational activities. A clinical interview approach is applied to understand the modelling strategies that are used by nine students during their three problem solving activities proposed in this research. Students showed different approaches in: their use of algebraic symbolism, the justification of their reasoning, representing their ideas mathematically. The findings can help teachers design school activities that are sensitive to the students' initial conceptions, in order to promote their mathematical modelling competencies.
