Install OS X on external drive without rebooting?

I manage several hundred MacBook pros at a school. There are times when I need to remove the hard drive from a machine, attach it to an enclosure and plug it in to my main machine via USB and erase the drive and reinstall the OS. I hate having to stop working for about 30 minutes while the OS installs exteally. I know there are other ways to go about this like creating an image of a fresh install and then restoring the drive with that image, booting up the other machine via recovery partition or network recovery, or just using another machine that isn't mine all together. I already use deploy studio, net boots, and other methods as the primary deployment method, I'm just curious about this specific rebooting installation. Thank you!