Mac to my school´s Windows Server 2003 network

Hey all,
Today was a good day I joined a Mac to my school´s Windows Server 2003 network and was quite proud of myself. That is until I logged in as a test student and found that a pupil had access to all the other student's files and some other important locations on the computer.
Test student obviously shouldn´t have these and was specifically designed not to have any access but it seems that they could access to their peer´s directories as well and some folders that seem to be private. This is not a problem with the other PCs.
Any ideas what is wrong?
