The discourse on the legitimisation of school mathematics in mathematics education and educational policy presents mathematics as an empowering qualification comprised of mathematical knowledge and mathematical meta-competences such as arguing or visualising. However, from a sociological perspective, it is neither clear in how far these qualifications are indeed acquired in school, nor in how far these qualifications comply with the possibilities and restraints of mathematics education. This paper implements a sociological view on mathematics education following the work of Michel Foucault. It confronts the ideals which the legitimising discourse brings forward with sociological findings and argues that mathematics education has unspoken social functions such as disciplining and subjection and that these functions have to be tabooed in order to connect mathematics education to the ideals of liberalism and enlightenment, leading to a discourse on the legitimatisation of school mathematics which does not do justice to students, teachers and others involved in actual teaching.