We did almost the exact same thing a couple years ago. 13 locations all with their own SV9100. Analog trunking at the schools and PRI at the head office, no DIDs at the schools and IP-CCIS between locations.

Each building has its own facility code that the school district used, 11, 14, 17, 21 etc so for those we just made a four digit extension number with first two being the facility code and last two being the last two of the room number. Room 158 in building 17 becomes extension 1758.

Some buildings have facility codes between 01 and 09, for those we used 5 as a leading digit so room 147 in building 06 is extension 5647.

We didn't have DIDs to try to match up and they wanted the extensions to match as close as possible to the room number as we removed the intercom phones from the classrooms and replaced with the NEC 2 button digital sets.

The systems were installed over the summer of 2015 and the CCIS was setup last year. So far everyone from teachers to admin staff love it. It was a great project to be involved with.