Between 2015 and 2016 we conducted computer based tests with 177 French migrant students enrolled in primary, middle and high schools, to evaluate their mathematical abilities and their levels of literacy. With 26 of this group, we supplemented data from their results with interviews and exercises on paper. By comparing their scores to those of native speakers, we sought to establish the reasons for the gap in mathematical achievement that exists between these groups of learners. We have observed that academic language plays an essential role in this phenomenon but we have proved that this factor is not necessary linked to their proficiency in French. In addition, we have found other obstacles which hinder mathematical achievement of some migrant students. Consequently, considering the range of these factors, we have concluded that an evaluation of the specific needs of each migrant student has to been made so that each of them can receive the help that they require.