This study focused on plan-type secondary schools where classrooms and open spaces are continuous and aimed to clarify the actual use of open spaces. The research methods used were questionnaires, observation surveys and interviews. The results showed that the utilisation of open spaces in secondary schools and students’ perceptions of open spaces varied significantly by grade level. Many students in grade 3 indicated that they were concerned about the line of sight from outside the classroom. It was evident that it is important to design spaces that can adjust the environment, e.g. by blocking the line of sight.