sbiddle:chevrolux: I would look at doing budget IP phones (Yealink T19P are great!) around the school as opposed to analogue. The benefit I see with using purely IP phones is that they are essentially plug and play. Meaning when you want to add a phone your installer can just remote in to the system, add an extension, and then post you a phone. You go plug it in to the correct switch port (assuming a port based VLAN is used) and you are away.
The ones I am aware of in Auckland are Hitech. I'm slightly apprehensive to recommend them though because the last system I saw from them was a Trixbox which I thought had been dead and buried for a long time. But then again I may be competely wrong - they have a nice website and it seems active so things obviously work for them.
They still sell Trixbox (now just called Fonality) but you're paying some pretty steep per seat costs every month for it.
Ahh right. So they won't sell you a system and only do a lease arrangement? Probably not an option for a school.