Network and system admins are not there to make your life difficult. You need to find out more about how the system works from them, and discuss with them how to add a Ubuntu image to the system.
The boot management system is probably not stored on the network card's firmware, but is likely some sort of http://en.wikipedia.org/wiki/Network_booting firmware that looks for a PXE server running on your network to download instructions and possibly disk images from.
Without a network or system admin there, someone is going to need to figure out how to use your systems.
- seriously a year and a half of no sys/net admin at a school is pretty cheap.
- Windows XP was released in 2002, and it's now 2013 and 3 Microsoft OSes later. New Ubuntu installs will definitely be all around better in every way.
- Look into the Ubuntu variant of the Linux Terminal Server Project: Ubuntu TSP as a complete replacement for your PXE server and XP disk images.
- organise a DHCP server for the lab and/or whole network, static IPs are a painful to manage.
If all that is too much for the current staff, then either hire an admin (needs money good luck with that) or simplify and make more tedious and possibly less secure, your systems. By seeing if you can change the boot order in bios to make it boot from the HDD or USB disk before the network card.
If you make it boot from USB before the HDD then you can make a bunch of bootable USB sticks you can put in each machine before class, and remove them when the class is over.