Virtual Server per VLAN authentication

[Arnaud Forster]

Hello all,
First of all, sorry for my bad english; so I'm going to try explaining 
as simple as possible... so let me know if you dont undestand what I say
I found a post here which is nearly the same as my problem : 
http://lists.freeradius.org/pipermail/freeradius-users/2010-July/048002.html
I need to create multiple SSID in a school.
On my server, I can create virtuals VLAN and attrib a DHCP server for 
each. My APs are able to manag many SSIDs and I can attrib one different 
VLAN to each SSID. I made a test and, according to the SSID I connect, I 
receive a different IP addres
The freeradius on my server is version 3.04. It's connected to a LDAP 
server. Everything is working fine; people can connect to the SSID using 
their user/password. But now, I'd like that some users can only connect 
to specific SSID/VLAN ; for example, I'd like to create a Teachers SSID 
and only user belonging to group 'teachers' (in my ldap server) can 
connect to it.
Is possible to do that ? On my server, each virtual VLAN has it's own IP 
address ; could I use it for my authentication  ?
As i'm completely a beginner in the freeradius world,,, sorry if my 
question is stupid
Per advance, thanks to all for your help