As Joem pointed out, knowing the details of your machine & software configuration is critical for efficient troubleshooting. Since you say that OS X connects fine, and XP does also everywhere but at school, a couple of things come to mind based solely on that statement:
1) Does your school dish out IP addresses only to registered MAC addresses? I'm guessing probably not, since managing that would be an administrative nightmare for an organization as large as a school, but it's possible. If this is the case and you have Parallels configured to use Bridged Networking, that could explain it (since XP would be attempting to obtain a second IP based on its own MAC address). If you're using Shared Networking then none of this will matter.
2) Does your school use WEP encryption? Apple uses a different WEP text-to-hex password encoding algorithm than the rest of the world, so while a text password will work on one side (either OS X or XP, depending on the brand of the router), the actual hex sequence must be entered as the password on the other side. This isn't the case with WPA encryption, however.