We have cisco at school and I cant use my xbox360 live. I gave IT my MAC # but now it wont connect to network?

At first my live wouldnt work because of a MTU problem so I emailed the IT tech people my MAC number on my Xbox and they said they would fix it. Now when I try to sign onto live it wont connect to the network where it did before. We have cisco here at school but I figured giving them my MAC number would solve the problem but guess not. I also tried the wireless thing and it didnt work out. I think I have exhausted my options. Any suggestions???? I need help =(