Code:
do shell script "networksetup -setairportnetwork en0 SSID password"
Okay... I have an idea. I tested a bit here and it seems to work okay.
Paste the above in Applescript editor and change the SSID and password to your real info. Then click run in Applescript editor and it switches form the existing wifi network to the one in the script.
So just change the script to match the wifi info for the single user who needs his own wifi and save the Applescript as an application. Then put the application in his login items. This way whenever he logs in, no matter what wifi network it is on, it will switch him to this one.
So I think that fixes him up.
Now for the other users. Make another Applescript but change it to switch to their SSID and set it up the same way as an application that launches as a login item. Should work the same way. No matter what wifi network it is on at login it will switch over to theirs.
You will need to set this up as a login item for each of those other users.
Unless I have overlooked something, I think this will do what you want.
The downside is the wifi passwords are not encrypted inside that application and if one knows how to lock inside an Applescript application package they could see the password. But it sounds like in your environment this might not be a problem.
Now watch as soon as I post this someone will post a link to a simple application that does this for you.