Federation Wallonia Brussel from Belgium is engaged in a reform aimed at developing personalized support to sustain differentiation: an additional teacher is in the classroom for a few hours. This paper relates the experiment with 10 schools at the level of the 5-8 years old. Its objective is to document the evolution of representations and practices of differentiation and collaboration. Individual questionnaires were submitted to teachers one year apart. Analysis of data revealed a broadening of pupils' conceptions. The presence of an additional teacher in the classroom developed a collaborative dynamic and a differentiation beyond the classroom space. At the end of our investigations, it seems that the representation of teacher's working time doesn't integrate the collective dimension yet and constitutes an obstacle. [ABSTRACT FROM AUTHOR]