Abstract: This paper uses a unique policy change in Canada's most populous province, Ontario, to provide direct evidence on the effect of reducing the length of high school on student performance in university. In 1999, the Ontario government eliminated the fifth year of education from its high schools, and mandated a new four-year program. This policy change created two cohorts of students who graduated from high school together and entered university with different amounts of high school education, thus making it possible to identify the effect of one extra year of high school education on university academic performance. Using several different econometric approaches on original survey data, the results demonstrate that students who receive one less year of high school education perform significantly worse than their counterparts in all subjects, even after accounting for the age difference between cohorts. Overall, both in terms of individual courses and grade point average, four-year graduates perform five to ten percentage points, or approximately one-half to one full letter grade lower than undergraduates with one more year of high school education.