The school as an institution assumes that students' grades are constituted by their assessments. This paper examines the background of this presupposition and provides a micro-analytical perspective of the grading practice of teachers in German High Schools ("Gymnasium"). This paper conceptualises the theoretical framework of the research in educational measurement in discussion. It is shown that the measured assessment of students and the teacher's observations are linked. When grading, teachers construct their own assessments. This process is depicted in this paper by two forms of observations: self-observation within the context of written examinations and third-party observation within the context of final oral exams. (Contains 1 figure and 7 notes.)