What about the network settings? What part of them?
SysPrefs > Network > Show Built-in Ethernet > TCP/IP. Check what the IP address is, whether it's been changed somehow. I don't know what the settings are for your router or region, but you can get those from your ISP. In fact I'd endorse what cwa has already advised - contact the ISP that the school uses for help with the settings.
Umm, I wonder if this a permissions issue. By that I mean that you cannot use another computer (your school's) to connect to the Net using your own (home) phone line, you have to go through your school's connection account. Did you check this with your IT teacher at school?
[Example - I cannot take my computer to someone else' home to use their router and ADSL connection, unless I use their ISP account and password. It's something to do with portals and is controlled by the phone line companies. It may not apply in your country.]
I'm not actually using a router, its the Airport thing.
Yeah, it's still got to go through a router, only it's wireless (Airport) from the Mac to the router, which is connected to your phone line. Unless you're using WiFi, in which case it's out of my league. Do you or someone else in your family use a (different) computer and internet in your home?