We conducted an exploratory study about the relationship between teachers' knowledge and fourth-grade students understanding, in the case of fractions. We studied the content knowledge (CK) and the pedagogical content knowledge (PCK) of 53 teachers and quantified their teaching experience, their mathematical preparation and the socioeconomic and academic background of the schools where the study was conducted. We applied one test at the beginning and one at the end of the school year to 1.532 students, identifying the gains and achievements of each group in the teachers' classrooms. The constructivist-oriented subcomponent of the teachers' CK showed a significant association with student learning, although it is less significant than the association with the teacher's experience. Socioeconomic factors were strongly associated with student achievement, confirming the strong divisions that characterize the chilean education system.