Back to basics - pull a machine out put it in a non-GPO OU - image it again run it up (using WDS?? sorry no your using FOG) then apply each GP to the OU in turn ... make sure it complies so that you know it is not loop or something.
Also just thinking > If your server is not 2K8 (RC2 x64 even better) have you got an admin Win7 box with admin tools to write your GPO's??
>assuming nice clean sys-prep'd images etc.. (maybe roll em out again ... students are probably creaming them any way)
>assuming was working before (what changes have subsequently occurred)
You would need to give more details of your system for much more help - hope this drops some pennies for you