School networks often mean firewalls and if you're running the second service pack, then the internal firewall (of XP) is by default active.
Go to Tools, Options, Bug reports. Click for an example and scroll down to the part were it says "Recieved incoming this session". A "True" indicates that firewalls aren't blocking LW and the problem is elsewhere. The oposite is true if it says "False".
Click
here to learn how to configure firewalls.