Whoa whoa whoa -- I think you've gotten yourself confused with terminology.
PCI is the internal bus in a Power Mac -- the cards are a minimum of 5" long, and go inside the computer.
The cards for laptops are
PC Cards, and are about 2x4", and very thin.
There is no such thing as an Internet card, even if that's what your school calls it. There is Ethernet (wired networking), and there's wireless networking (802.11b and 802.11g, which Apple calls AirPort and AirPort Extreme, respectively).
I assume we're talking about an 802.11b or 802.11g system here, and I am furthermore going to assume that your school is using the 95% of systems that are fully compatible with the industry-standard Apple AirPort Extreme card that you could fit inside your PowerBook. In this case, just get the AirPort Extreme card installed in your PowerBook, and be sure you're running Mac OS X 10.3, which adds the ability to join several types of industry-standard password-protected networks that 10.2 cannot join.
If, on the other hand, your school is running one of the few proprietary wireless systems (they're 802.11, but using their own non-standard password protection schemes), you may have no choice but to get a card from their vendor. Since your computer doesn't have PC Card, you will need to see if the vendor sells a USB wireless interface, because USB->PC Card devices do not exist. (This isn't quite true, actually: they do exist, but they only support PC Card memory cards. A networking card might physically fit, but it wouldn't work.)
If your school uses a proprietary system, be sure to send a note to upper management making them aware that their computer guys selected a proprietary system that may cause them to be locked in with that vendor.
What vendor card do they want you to use?
tooki