raschemmel:
For a school I would think it would have to be differential RS485 (CAN BUS or MOD BUS)
CANBUS is 1000 meters (3280 feet @ 50kbit/sec
http://digital.ni.com/public.nsf/allkb/D5DD09186EBBFA128625795A000FC025
MODBUS about 1200m for speeds less than 100kbit/s
RS-485SparkFun Transceiver Breakout - RS-485 - BOB-10124 - SparkFun Electronics
http://forum.arduino.cc/index.php/topic,17118.0.html
Here's a tutorial example for RS-485
http://arduino-info.wikispaces.com/SoftwareSerialRS485Example
http://yourduino.com/sunshop2/index.php?l=product_detail&p=323NOTE THE RESISTORS AT THE BOTTOM OF THE SCHEMATIC.
http://yourduino.com/sunshop2/index.php?l=product_detail&p=323The 120 ohm resistor is called a Termination resistor. There needs to be one of these at the two extreme ends of the bus.
The 470 ohm resistors are biasing resistors to keep the signal centered in the 0 to 5V range
RS485 will not work without the termination resistors to prevent reflected wave.
The alarm panels are fairly dumb devices. They just need to transmit 7 digit codes when they are armed/disarmed, and receive 10 character names when a zone is faulted. I think anything over 1kbit/s would be sufficient.
EDIT: you added some more since I originally read the post.
The termination resistors for r485 were what were worrying me, since this use case wont match that model. There will be many (say, up to 5) devices that all connect back to a central point over varying distances. I was thinking that some type of rs485 hub would work.