I'm not sure I understand the question? If the carrier issues a range of say nnn-1000 - nnn-3000. And you want internal extension 4500 to be nnn-1000 you would just program it as such. Did's don't need to match internal extensions. In his case being a school he is probably porting existing numbers at least for the main numbers that are probably not going to be in any special sequence.
I have 57 nodes at work, with another half-dozen to be added this year. Not all have DIDs, but for the ones that do, I match the last 4 digits of the DID with the extension number. With 700 DIDs at HQ alone, it requires close coordination with our carriers.
