If the default calendar is the school calendar it could be that the Parent calendar is not subscribed to this default one.
Go to Calendars > Select School Calendar (Staff) > Select Preferences (Top right) > Subscriptions > You will notice that the Parent Calendar does not even appear as an option. This is because the Parent Calendar is set to private.
Go to Calendars > Select Parent Calendar -> Select Preferences > Set View Privileges to Shared: All logged in users -> Save.
Go to Calendars -> Select School Calendar (Staff) > Select Preferences > Subscriptions. You will notice that the Parent Calendar will now appear. Select it and select Save. The Parent Calendar should now show on your Default Calendar