There are a couple of things you could try, a bit tedious however setup a scheduled task on every machine to shutdown the pc at a certain time.
Or providing you are a domain admins on the server run a batch file from the server to shut the pc's down remotely change computername to the pc name and copy the line for every pc you would like to shutdown.
something along the lines of
shutdown.exe \\computername -s -t 60 -f
if you add the -f switch aswell this will force the shutdown incase programs are crashing in the background just remember to make sure no one is sitting at the pc!!
I would test this on a dummy pc first before running on the server so you get the hang of it. If you getinto any trouble click start run shudown.exe -a which will abort the shutdown this is why you give it 60 seconds to allow you to do this.
This script could also be scheduled to run on the server if you are not around when it needs to be run.
Hope this helps.
Lee