This paper presents a mathematical model of the flow of students enrolled in a college program through the curriculum. From a simplified model of the curriculum it is possible to estimate the number of students in every level, number of graduates and graduation times of a specific program or a set of programs. Some related phenomena, such as cancellation of courses, repetition, approval, registrations, transfers (and dual degrees programs), loss of student's quality and graduation are modeled. The model has been validated with historical data from the School of Engineering of the National University of Colombia. The model analysis and several simulations allow to understand the effects of the modeled phenomena on the total number of students in a curriculum and on their graduation times. Using the model there is also a partial explanation regarding the increase in the number of students registered at the School of Engineering since 2009.