The relevance of the study is related to the need to increase the validity of formative assessment in classrooms, which is a factor in improving the quality of education and the development of students. The purpose of the study is to determine the criteria for the validity of formative assessment, which will allow gaps in its implementation in national schools and provide professional support to teachers to improve practice. The research methods used were lesson observation (the analysis of teachers’ approaches to formative assessment of students’ achievements) and a questionnaire, which revealed similarities and differences in the views of teachers and students on the practice of formative assessment. The results showed that the experience of formative assessment of teachers is associated with their knowledge and skills of implementing formative assessment. However, it also became known that the knowledge and views of teachers about formative assessment does not always correspond to their practice. The results of the study will allow us to develop guidelines for the implementation of formative assessment and ensuring its validity, which will contribute to improving the quality of education in schools in Kazakhstan.