Training through the Internet poses a series of technical problems and pedagogical issues. Traditional training is not indiscriminate but takes on different forms according to the needs of the subject being trained and the context where such training occurs. In order to make the systems adaptable in this way, a model of the student ￠ € s characteristics - the student model - has to be set up, maintained and updated. However, there are many difficulties involved in obtaining sufficient information to create an accurate student model. One way to solve this problem is to involve students in the student modeling process, stimulating them to provide the necessary information by means of a dialog in which the student and system build the student model according to a collaborative process. The present work describes a cooperative student modeling method (Cooperative Student Assessment - CSA) which builds a joint system-student assessment of student ￠ € s activities on the basis of the student ￠ € s self-assessment ability estimation and a prototype system for children, addressing the learning of fractions, in which CSA is implemented. The article also reports the result of an experimentation carried out with learners attending primary school aiming at evaluating the effectiveness of involving students in the assessment process by comparing two versions of the same system: one using cooperative student modeling and the other the traditional overlay model.