The aim of this study is to establish to what extent teachers' knowledge, in association with school socioeconomic level, students' previous knowledge and level of mathematical knowledge achieved in school contribute to the knowledge that fourth-grade students reach in conceptualizing fractions. Information was obtained from 328 fourth-grade students of nine schools and their respective mathematics teachers. The results show that the 77% of variability observed in the conceptualization of fractions could be attributed to student-level variables, while the remaining 23% would be attributable to school-level variables. On the other hand, 38% of the intra-school variance could be explained by students' previous knowledge, and virtually all the between-schools variance would be explained by the academic level of the school or, in 32% of cases, by the socioeconomic status of the school. Teacher knowledge, alone or in combination with other factors, accounts for about 10%, with a significance of 10%.