Well, you could try changing ports but there's no guarantee that this will fix things.

Go to LW's tools > options > advanced > firewall config. Change LW's listening port to one in the range of 49152 - 65535. Apply. Now change your Norton firewall config so that the new port number's allowed thru the firewall.

If that doesn't help then you'll need to find out about the school's p2p policy.