SFUSD Google accounts can only send and receive emails from SFUSD teachers, so they cannot receive calendar invitations from normal gmail accounts, like the gmail accounts of their parents. So you cannot invite them to join a calendar as usual.
Here's a workaround.
- Create the calendar to share from a normal gmail account.
- Go to Calendar Settings / General
- Find and copy the URL under "Secret address in iCal format"
- In Google Calendar, in the left pane, go to Other Calendars / Add
- Select From URL
- Paste the URL into there.