Lemme repeat... parents can't cop out on their responsibility of authority! It's turning the job of nanny over to the teachers that has brought on much of the moral rot and destructiveness of today's children. Yes it would be a good idea for teachers to explain at the start of the school year that the pledge is not required if the parents or guardians do not wish it, but to make this the main point is to miss the main point.
This isn't really adequate -- the instructions of the parent of guardian ought to be explicitly obtained, and the teacher ought to instruct the class that the school authority does not extend to this point.