There has been renewed policy interest in intergenerational social mobility as a route to a fairer society, but in ignoring the sociological evidence this article will argue that the current policy agenda will fail to achieve its goal. Based on an analysis of "social congestion", "social exclusion", and "social justice", it also argues that existing sociological research on education and social mobility needs to be extended. In the early decades of the twenty-first century, the experiences of working-class and middle-class students and families are not defined by intergenerational social mobility, but by social congestion and an opportunity trap.