When you're setting up a computer for lab use, it makes sense to use auto-login and one-button shutdown to automate start up and power off. Below are the settings required to do this:
Use the Sysinternals tool autologon to pick the user who will automatically log in.
Local Security Policy > Local Policies > Security Options > Shutdown: Allow system to be shut down without having to log on.
Local Security Policy > Local Policies > User Rights Assignment > Shut down the system > add "Users".
Virtualbox can also start up and shut down VMs automagically on login.
Auto start up: vboxmanage startvm <name> --type headless
Controlled shut down: vboxmanage controlvm <name> acpipowerbutton
Just use the vboxmanage command line:
Task Scheduler > Log on task / Log off task OR
Group Policy > User Config > Windows Settings > Scripts (logon/logoff)
In addition,
Ensure that the latest vbox additions is installed.
Ensure that the default ACPI power button action (in the VM) is "Shut down".
Alternatively, if you want to set up your computer like a server (VMs start on boot, rather than on login), then follow this procedure:
http://www.irongeek.com/i.php?page=security/madmacs-mac-spoofer