No, you can't change that field (it gives me fits, too, but for different reasons--I really wish we could get rid of it). What you can do is change the text that the volunteer sees on that screen. If you go into edit mode, and fix "navigation" so that you are seeing what the volunteer sees when they fill in that screen, you will see the standard text of "Please complete the fields below. Keeping your contact information up-to-date helps us keep you informed with what's happening at our school. You can come back at any time to update your information. " --unless you have already overridden that. Anyway, where it says that, you could add something like:
INFORMATION ON THIS PAGE IS USED TO PRODUCE THE SCHOOL DIRECTORY. IF YOU DO NOT WANT TO BE IN THE DIRECTORY, PLEASE MAKE SURE THAT YOU CHECK THE BOX THAT SAYS "DO NOT INCLUDE IN SCHOOL DIRECTORY."
Your other option might be to have a separate opt-out form. You probably won't have many people opting out, so if you cross reference the forms you receive with the data in the system and research the discrepancies, that should catch everyone.
Or, just before you print the directory, you could send each family a printout of their information and ask them to verify it, and give them a last opportunity to then say they don't want the info published.
on our forms, everyone that fills out the form will be added into the directory unless they check the box. since the majority will choose to be added, the item is there for the few that do not wish to be added in.