Formative assessment is an evaluative practice developed in the classroom for the improvement of learning using evidence on student progression. The objective of this research is to compare sample groups from multigrade and single-grade classrooms on the theme of formative assessment based on the students' opinion of the teacher's performance. The method used was a comparative quantitative method. The sample type is a probability sample of 683 students from 5th to 8th grade from urban and rural schools in the commune of Longaví, located in the Maule Region of Chile. A validated Likert scale questionnaire with a high level of reliability (α = 0.93) was used. The results of the research showed that, in the six dimensions, the best teacher performance concerning formative assessment is found in multi-grade schools and not in single-grade schools. This can be explained on the basis of several reasons, among them the level of adaptability that teachers have in this type of classroom, the heterogeneous characteristics of the classroom (different ages and learning goals) and the need for teachers to monitor the learning progression of students with different classroom characteristics.