samba - Aug 2008 - Connecting to OSX machines with user name/passwd different than login name/passwd

Sorry for the long post - but this is a unique situation as the clients 
(iMacs) are controlled by our central school district IT dept. As the 
local tech guy at a high school I'm allowed some control - creativity, 
but not much.

End goal:
Setup a linux file server using samba to connect to tightly controlled 
macs in a school setting. I said tightly controlled because the district 
IT folks control the OSX image that is loaded on all macs at our school. 
The issue is the space provided to students (on the district servers) is 
very limited. For specific classes I want to have a local linux server 
for students to save large files.

OSX environment:
Students will login under a generic login as    Student  --- no 
password. Every student has a unique 8 digit ID (and currently they use 
their ID and password to access the districts own servers).  I will not 
be able to alter network settings etc. on the iMac clients - loading 
special SW is possible but not desired.

Linux environment:
Ideally I would create a an account for each student ( their ID and an 
assigned passwd) so each student would have their own file space to 
store project files. If this is not possible I could go with a single 
shared space --- but you know someone will eventually delete someone 
else's files.

Current  Progress:
I have an Ubuntu file server running  samba. Samba is setup using tdbsam 
(thats just the way I set it up) and I am able to connect to both a XP 
machine and a mac (this is at my home so this is a standard OSX load). 
However, I can only attach to samba if I use accounts with identical 
user names and passwords that also exist on the linux server - XP 
machine - mac. So all 3 machines have a user called  Bert  and all have 
the same password for the Bert account. If I have an account named Sam 
on the linux server and try to get to it from the Bert account (on the 
mac) I get an error ---- "The alias "my server name" could not be
opened, because the original item cannot be found". I am attempting to 
connect on the mac using the  Go -> Connect to Server ->    then browse 
finding the server under the workgroup name its broadcasting. How can I 
get around this need to have identical accounts/passwds on both the 
clients and the server ?

Thanks for any help - tips in advance

Henry