The best way to do it is to launch ZBrush on each computer and use Zplugin>Deactivation>Web Deactivation to shut down that copy of ZBrush and immediately return the activation to the serial number. It will launch a web page that asks for the email address associated with the account to complete the deactivation. No serial number information is given out, so you could have your students do this at the end of their final session.
After deactivation you can then reinstall everything as normal.
You should install and activate while logged into the computer as an administrator. This sets up the permissions so that all students are able to run the program. If you are using Windows Vista or 7 you may get a message letting you know to relaunch as administrator. If that happens, right-click on the ZBrush executable and choose Properties. Go to the Compatibility tab and check the option at the bottom to run as administrator. Apply, then close the window.
To activate you will need your serial number and the email address associated with the account. (For security reasons, this information is never available from within ZBrush or its folders. If you no longer know that information please submit a support ticket or call our support line so that we can assist you.) Launch ZBrush and choose Web Activation. This will launch the browser to a page that asks for your serial number, the email address associated with the account, and a computer description. It helps to use meaningful descriptions such as a unique machine ID number like the lab’s desk/seat number.
If you get a message saying that all of the activations for the serial number have been used, that means that there are some copies that did not get deactivated. When that happens the system will also list the machines currently counting against the serial number, using the date of activation and computer description. (This is why it helps to give meaningful names at activation. :)) Check off the old activations that should no longer be counting against the serial number, submit the form and then confirm the deactivation. This creates a support ticket so that we can manually adjust the serial number to show the correct activations. We do this a few times daily. If you need it urgently, wait until you’ve received a confirmation email with the ticket ID number and then call our support line. We can use that ticket ID number to immediately pull the ticket and process it for you. Please don’t call before you have that confirmation email as the ticket won’t be available for action in our system yet.
That should cover all possibilities. It’s pretty straight-forward. Hope that helps!