When SchoolCal is launched, it creates a list of all the calendars currently in your iCal database (the Calendar Store), and presents them to you in the popup button near the center of the Main Tab.
You can select ANY calendar in your store to receive the dates once you are finished defining them. Conveniently, you can also create a new calendar without launching iCal:
Selecting Create New Calendar... brings up this window, where you enter the name of the new calendar:
Your new calendar becomes the new default calendar, and the next time you launch iCal, that calendar will be part of your regular collection of calendars.
If you subscribe to any iCal compatible calendars, they will be Read Only, and you will not be able to write Events to them. As you can see in the image above (the list of calendars -- US Holidays), any Read Only calendars will be identified as such. You can choose to simply not show those calendars in Preferences.