I know such situations and would like to answer ' don't try to solve a social problem with technology.'
1. Why didn't you create separate courses per teacher
Typically a teacher is teacher on course level. This means all elements are visible for the teacher.
You can add the teacher as student to the course, prepare a topic per teacher with some activities and give the teacher teacher permissions for this elements
Its not possible on topic level.
Now comes the trick. Create a group for all students. Define that the teacher topics have limited access to the teachers topice. As long as the teachers are not in the students group they will not see the elements.
Hope its clear.