Do you mean the students can see topics that only teachers are meant to see and not students? Do you mean actual courses just for teachers and not students? YOu need to put an enrolment key on the courses you don't want students to have access to (in the course administration block>settings>enrolment key) Look in the assign roles link of the course administration block too and take out any students you don't want there. Also - make sure that your students are not assigned the role of student site wide because that will make them appear in every course everywhere. Go to your front page
to site administration>users>permissions>assign system roles and if there are students in there, take them out! Also - while you are there - if there are teachers in there too, take them out. Only put them in the courses they work in.