If you need to run Symantec Live Update through SecureSchool, you need to make some Authentication Exceptions, since Live Update runs as a service and does not provide any authentication information to SecureSchool.
SecureSchool, ISBossBox, LibraryDoor, Symantec Live Update
To add Authentication Exceptions, go to "Website Filtering" -> "Authentication Exceptions" -> "Add A Site". Add entries for:
Once you add those two sites, go to "Commit Changes" and restart. Symantec Live Update should now work.