I have the same problem. I haven't figured out how to get online at school using knetworkmanager, but I can do it via the command line:
[root@tatooine ~]# /sbin/iwconfig eth1 essid UCF
[root@tatooine ~]# /sbin/dhclient eth1
(where "UCF" is the name of my school's wireless network.
If you find yourself getting kicked offline repeatedly, kill NetworkManager/knetworkmanager until you get home.
If anyone knows how to make this work using knetworkmanager, I would love to hear about it. I'm always showing off my linux laptop, and it kinda takes the uumph out of it when I have to use the command line to get online.