One of the most important goals of teaching is to enable students to take responsibility for their own learning. In mathematics lessons guided by tasks, co-constructive solution processes of students in partner or group work can be decisive for that goal. The project DISUM aims at investigating how students and teachers deal with demanding mathematical modelling tasks. Among other things, pairs of students with different abilities — partly with, partly without the presence of a teacher — have been observed in laboratory settings while solving such tasks. In the present paper we point to four types of problems which can occur in student-student or student-teacher interaction during co-constructive solution processes. As an example, we have chosen the „Sattelfestr” task. It becomes clear that the broadly postulated claim for students’ self-regulated learning raises considerable demands for teachers and students which, up to now, have not been sufficiently investigated by research into teaching and learning.