Roles and permissions

I'm not understanding something I think. What I'm trying to do is create a hidden category that only some students can see, the majority can't. It contains sub-categories and courses. This is the way I've tried to do it:

a) created a duplicate of the default Student role, edited it to "Allow" - category:visibility and nothing else;
b) renamed the duplicate category, "BoM";
c) assigned all the students I want in the global assignments to the new role of "BoM";
d) hidden the top level category, but left all the sub-categories and courses underneath them not hidden;
e) as administrator, I go into a student user profile, and select "login in as" and go back to front page.

As the user, I can't see the hidden category.

What am I missing here?

I'm running 1.8.3+