Rubrics are assessment tools that help students gain complex competencies. Our pilot-study aimed to evaluate an assessment instrument supporting teachers to teach and assess in alignment with national standards. Students in four 5th grade primary classes (n=67) in two Swiss cantons worked on their mathematical reasoning competencies using our standards-based rubric. An achievement test was administered at the end of the study. The results of quantitative and qualitative analyses demonstrated that the stages of the rubric were valid and the instrument was reliable. The tools’ expected learning support features were confirmed: Working with the rubric fostered the teachers’ and the students’ understanding of the standard, enabled the students to self- and peer-assess and allowed teachers to provide effective feedback.