Can instructor create private forum for his circle?
How?
Create a private group and attach a private forum to this group. This forum can only be accessed by members of this group.
But it may be a complicated, because when new user register with instructor than instructor have to add him in that private group and it will repeat for all his new student.
Except of that
If there is functionality like
Instructor make a forum and topic than all students who allowed by instructor can see that forum and another student cant see that.
In that case you have to add the new user in that private group manually .
You can use the same private group feature to have a forum for the students that you like to have in that private group forum .Just create a private group and invite the users .