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
MODBUS about 1200m for speeds less than 100kbit/s
http://www.innovatic.dk/knowledg/rs485/rs485.htm
http://forum.arduino.cc/index.php/topic,17118.0.html
Here's a tutorial example for RS-485
http://arduino-info.wikispaces.com/SoftwareSerialRS485Example
NOTE THE RESISTORS AT THE BOTTOM OF THE SCHEMATIC.
The 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.
MAX485.pdf (440 KB)