The interdisciplinary research project Co2CA investigates how assessing and reporting students’ performances in mathematics can be arranged in every-day teaching in such a way that teachers are able to analyse students’ outcomes appropriately and organise further learning as well targeted as possible. In this context, 39 classes of German middle track schools were observed for several weeks while dealing with mathematical tasks focusing on technical and modelling competencies. Based on the assumption that assessing and reporting students’ outcomes regularly will foster learning processes, students from some classes were given individual, task-related feedback, in some classes several times in a written form, in some classes in addition permanently accompanying the students’ solution processes. In this chapter, we describe the study and report some preliminary results.