This study analyzes the effect of opening a new high school on pupils? allocation to different tracks at the end of lower secondary education. We use high school openings to highlight the constraint local school supply exerts on individual schooling decisions. The working sample covers all pupils enrolled in 9th grade between the school year 2007-2008 and the school year 2010-2011 in France. The estimation strategy (a generalized difference in differences) takes advantage of the variation in time and space of the openings of high schools to estimate the causal effect of an increase in school supply on the allocation of pupils to tracks at the end of 9th grade. We show that opening a new high school increases the probability to continue in high school for pupils from the nearest middle school. This increase is driven by the vocational track, and by low achieving students.