When Editing an Assignment, there is a field "Visible to Students" near the Points Possible field.
This is the same as the Show/Hide to students on the Grade Centre column and directly toggles that setting ie Yes = Grades visible to students; No = Grades not visible to Students.
Changing this directly changes the setting also accessible via the Grade Centre of Show/Hide to students.
This does NOT control the visibility of the content item/link to students.
We acknowledge this is potentially easily confused as relating to the content visibility and are working to address the wording &/or layout to make it less so.
Control of content visibility to students is this item further down the page.