In this paper characterizations of mental objects for fractions of middle school students (from 12 to 14 years old) with absenteeism problems and low academic performance, are described. A test was designed as part of a research whose general purpose is to contribute in the building up of better mental objects for fractions through a teaching sequence. Items for the test were structured according to the results of a didactic phenomenology of fractions and the curricular content proposed for the last years of elementary school. Results indicate that students have a greater success in issues related to continuous model partitions phenomena, whereas they are less successful using fractions on the number line. Students cannot transfer their knowledge from an area model to a linear model to identify or represent fractions. [This paper is written in both Spanish and English. For complete proceedings, see ED581294.]