The introduction of all day-schools in Germany was due in part to the results of the Programme for International Student Assessment (PISA) study 2000, which revealed a striking social inequality in the German education system. It was expected that in all-day schools especially "at risk" groups would be supported and thus the gap in achievement based on socioeconomic status (SES) would be narrowed; however, few studies have explored this. In this paper the potential of all-day secondary schools is investigated through analysis of data from a nationwide study on the development of all-day schools (StEG [Studie zur Entwicklung von Ganztagsschulen]). The findings support the idea that all-day schools could help to narrow the gap between low and high SES students in several ways.