Recent social scientific research on penitentiaries has shifted toward the way young inmates experience incarceration in order to analyze the socialization produced by disciplinary measures in prison. Until then, most studies had dealt with juvenile issues. This paper investigates the socialization processes of the inmates in “établissements pénitentiaires pour mineurs” (EPM – juvenile prisons) By placing group time at the heart of its detention system, this “new” prison is a good place to study the ongoing socialization process. Partially re-appropriated by the jailed teenagers through what this paper refers to as “surcodage sexué,” these group times lead to exacerbated displays of gender, which are often prohibited but inherent to the configurations generated by the new system of detention promoted by the EPM.