From some searching it looks like you need to register your xbox with your school's network. Most school networks will only allow registered to devices to connect. On your computer they usually just re-direct your browser to a registration page the very first time you connect, but this obviously won't work with a console. You should contact your helpdesk or IT department to get the MAC address of your Xbox registered. I found all this information here.
http://www.xboxlivetheguide.co.uk/XB...e=MTU%20Failed
http://www.xboxlivetheguide.co.uk/XB...ct%20to%20Live
I would NOT recommend following the directions in the second link and cheating by copying your computer's MAC address to your Xbox or making your computer look like your Xbox to register it, because you could really anger your IT department. The best thing to do is just contact them. I worked for the helpdesk while I was in college and for the first couple years when we ran into that situation for a student we actually did recommend that they make their computer temporarily look like their xbox and register that way, but later we just had them manually fill out a form so we could register the MAC address. Who knows what the policy is at your school, so just ask.