Understanding by Design (UbD) is a well-known curricular methodology aiming at leading students to develop a deep understanding of the arguments proposed by teachers. Through a path including stimulating questions, student motivation, deepening of the argument, reflection, design and assessment of authentic performances, the students develop a broad understanding of the learned argument. While there are plenty of articles describing various experiences in classrooms and schools showing the benefits of UbD, rarely has this methodology been the object of experimental or quasi-experimental studies. A study conforming to quasi-experimental criteria has been performed in a Swiss vocational school, where a teacher taught two groups of students using UbD principles, while using with a third group a more usual teaching approach. The curriculum topic was the study of mathematical relations and functions. The results, collected during one school year, showed that the first two groups outperformed the third and that the effect of UbD lasted at least for one school year. Since the teacher remained the same, we can likely attribute the outcome to the use of the methodology.