In Geneva canton (Switzerland), a special course has been designed to deal with problem solving. Within one 45-minute period per week, teachers have to improve students' problem solving competencies and to assess them frequently. In order to assess students' problem solving competencies, most teachers use grids of criteria with a summative purpose. The global objective of our research is to find out if and how using such a tool can also foster formative assessment processes. In this paper, we present an exploratory study focused on teachers' formative assessment practices. In order to do so, we study the practices of a teacher giving this IBME-centered course and show how she uses formative assessment in her teaching practices.