Three teachers and a researcher have co-designed a teaching activity intended tosupport students’ learning of two strategies for subtraction. The researcher hasput focus on the relation between theoretical principles, introduced to underpinthe participating teachers’ work, and the learning outcomes of their 33 studentsin grade 4. The principles are adapted by the researcher during three designcycles and negotiated with the teachers to meet emerging needs in the designprocess. The three teachers are fully responsible for planning, implementing, andevaluating an iterated teaching activity designed according to these principles.This study indicates positive effects of targeting low-achievers with teacher-ledstructured group activities, using guiding principles from self-regulation theory.