This default calendar is the South calendar, and you can display both the South calendar and your classroom calendar. Edit the page, click on the wagon wheel gear for the calendar widget, then click on Add Another Calendar and you can select from any Google calendars of which you are the owner. Make sure you have allowed your calendar to be viewed publicly.
If you accidentally delete the widget and need to replace it, you can Go to Insert, then Calendar (third column) and select your calendar to insert.