Serial port not listed when on school network

For some reason I'm able to upload to my mega 2560 on my mac (10.6. 8 ) when I'm on my home network but when I bring my laptop to school the usb port (/dev/tty.usbmodem411) isn't an option on the tools>serial port menu. I've tried turning of my airport but that just leads to the error "Serial port /dev/tty.usbmodem411 not found. Retry the upload with another serial port?" Only the Bluetooth-PDA-sync and Bluetooth-modem ports are available in that menu.

Thanks in advance