Disable USB drives for specific users

Hi
I need to prevent a small group of students from using USB drives but allow them for everybody else. Clients are XP pro on Server 2003 domain. http://support.microsoft.com/default.aspx/kb/555324 explains how to disable USB drives but this only seems to work if I apply the policy to the OU containing my computers. This doesn't work for me as I have different students using the same computers, but only want the use of USB drives to be disabled when certain students logon.
Any ideas?

Thanks

Jason