The research based on instrumental variable technology shows that high school education is of great value to potential dropouts, but for students who will continue to receive college education, we know little about the benefits. In order to help fill this gap, I use the recent middle school reform in Ontario to measure the value-added of one year's high school mathematics to students preparing for college. Despite the existence of self-selection, the particularity of the theme of this reform makes it possible to determine the benefits of studying mathematics for one more year: people can use the theme unaffected by the reform to control the potential ability difference between the control group and the treatment group.