John,I think it more likely they selected a zip code that would be somewhat in line with those in the general area of the make-believe school.
I also think using the school as a return address was something only an American (or person very,very familiar with Americans) would do.They would know,in our culture,children are important,and that a mail clerk would be likely to open a letter which appeared to come from school children.
In "nomadic" cultures-such as those which founded the Wahabbi (?) sect of Islam, elders are MUCH more important than children.If resources are scarce,they are more likely to be allocated to an elder than to a child.By extension,a letter received from what appeared to be children would probably be discarded-unopened,as it would likely contain a request for help or some sort.
I propose that the ZIP code was chosen first, to represent the date of founding of the Directorate of Plans, and then the city was chosen, probably from a map, as being in about the right area.
The alternative hypothesis is that the city was chosen first, but then one has to ask two questions: Why Franklin Park? And why the incorrect ZIP code? It's a lot harder to see how they would have gotten an incorrect ZIP code for a city than to see how they might have gotten an incorrect city near a ZIP code.