Hanging on log off

Hey Everyone,
I work in a school system. We use Windows XP, 2000 and in the Elementary
schools we use Win 98 machines. We have a server 2003 domain, upgraded from
NT4.0 this past summer. Each grade, K-5 logs on as one user when a class is
in the computer lab, i.e. there might be twenty some students that log on as
CG3, which would be grade three. Now this works fine in three out of the four
elementary schools, but one it does not. They logon and work just fine, but
when they all go to log off at the same time almost all of the computers
freeze. then you have to wait several minutes before you are able to log them
off. This was happening with NT4.0 also; any ideas?