Faced with the diversity-efficiency dilemma, private companies apply "mass customization" strategies to add diversity without adding costs. As schools are urged to become more "customer oriented" they also face a diversity-efficiency dilemma. This article asks how Dutch secondary schools cope with this dilemma and to what extent they apply "mass customization" strategies. A careful selection procedure aimed at a maximum variety of school practices resulted in seventeen schools for which case studies were conducted. Data collection included written material, observations and interviews. Analysis of the combined data indicated six dimensions along which schools differentiate their educational offerings. On the basis of emerging patterns of differentiation, four categories of schools were distinguished. These categories appear to be closely linked to organizational strategies pursued by schools. The article concludes that practices adopted by schools to cope with the diversity-efficiency dilemma strongly resemble mass customization strategies applied by companies producing tangible goods. In the final section, the risks and inherent contradictions of these strategies are pointed out. For while government policies and schools seek to put the needs of individual students at the centre, the inevitable diversity-efficiency dilemma may cause many schools to adopt practices students never asked for. (Contains 6 notes.)