You can manage access to Scheduler feature in Add-ons (Security in pre 4.0.5 versions) section under System Administration section:
Use the drop-down menu hidden under the cog icon (1) to access "Add-ons" (2) option.
"Access Manager" (3) is located at the left bottom area of the page.
You can grant permissions to Project Leaders, project roles, individual users and groups.
Only entities (groups, individual users and projects) who have permissions, are able to see and access 'Scheduled Issues" option in the left hand side panel and "Schedule Issue" option hidden inside "more" menu:
Only user with granted permissions are able to see and access "Schedules Issues" (3) option in the left hand side panel. The same goes for "Schedule Issue" (2) option cloaked inside "More" (1) menu.