It isn't possible, at the time of writing, to subscribe to an internet calendar in Outlook for Mac (version 16).

You can open shared, resource or meeting room calendars. 

From the left hand menu select the calendar icon in the bottom left corner.

From the toolbar select 'Open Shared Calendar'.

In the top of the box that appears start typing the name of the calendar the list below should start to auto populate.

Select the calendar/users and click 'Open'. 

URLS for the calendars;
School holidays
PE Fixtures

Staff can request the Staff URL from the helpdesk.