Hi @FeliciaLasmana this is quite a serious issue as they are likely to be able to see aspects of students and class work that are reserved for the teacher, which may result in breach of provacy, confidentially, data protection and assessment integrity.
As a starting point I suggest that you check the course roster (people view) and what their enrolment status is. You may be able to update or disable this here, or you may need to contact your IT support.
It is possible that they were assigned observer status at sub-account level (or has another similar status) - you may know or may need to ask IT.
Finally you may need to check / ask IT to check, whether the permission settings are correct across the various roles.
We'd be interested to hear what you find ....