airport ethernet cable 'not plugged in'

[Deanneg]

Hi there, hope you can help, and the more you dumb it down for me, the more I'll appreciate it! =)
I am in a school with three airport express base stations. In one area of the school, the airport is working (I get signal), but it does the whole 'self-assigned IP address'. I also checked in airport utility and it says that airport isn't plugged in (to the ethernet port or whatever), but it is. So I changed the cable. I've changed the whole airport. The airport works in other parts of the school, but the airport swapped doesn't work in the problem area.
Any ideas of what to do next?
So far, I've:
reset router
reset airport (unplugged)
changed cables
changed airport

Thanks for any help given =)

 

[Satcomer]

So you are connecting the Airport Express connected to a school wireless network? If so you must setup the Express in "Bridged Mode" on the Express since the school's wireless is already doing DHCP. So factory reset of the Express and set it to join the wireless network.