It works with every other machine?
And your machine works with your home WLAN...
This can go back and forth as much as you like. You cannot categorically state that wifi is broken on Mountain Lion when you are having compatibility issues with ONE network.
WLANs of this size are generally very complicated and there are many hundreds of settings which could affect a wireless network's ability to serve clients. Sure, someone could have come down from a WLAN company and checked the settings and they're going to say the settings are OK - leave your ML machine with the technicians while they're looking at the network and you can bet your left nut that they'll find a setting on the WLAN controller which will suddenly make your laptop start working again.
On the flip side of the coin, I administer a WPA2 encrypted network at my work and the wireless has continued to work through Snow, Lion, and Mountain. It also works at home, and at any wireless network I have attempted to connect to.
Does this mean I'm lying because your machine won't connect to your school's WLAN that Mountain Lion's wifi is broken?
----------
Oh, and going back to this from the first post:
If you take the .kext from SL, and delete Lion or ML's kext, and put SL's kext in place, then the issue is solved. But when there is a 10.8.x update, it breaks it, and things go downhill.
Surely that shows you that the network has a setting which is not compatible with Lion or Mountain Lion. Your network engineers need to sit down with an affected machine (which you've said is all L or ML machines) and change the correct settings until the machine works fine.
Alternatively, you could just, you know, put the SL kext in (which you've done before and it works). There's also a really handy tip for updates too.
Install the 10.8.X update....
... replace the kext
Job done.