How do I recover documents from a lab or classroom machine? 

Generally speaking, it is not possible.  All computers in the school labs and classrooms have Deep Freeze installed for security reasons.  As a result, when a machine with Deep Freeze is restarted it will lock the hard drive and delete any locally saved items (Word documents, Excel spreadsheets, images, etc), reverting the machine back to a default state.  Recovery of your files is not possible once that occurs.

Where should I save my files when working on a lab or classroom machine?

Please don't save your files locally to a lab or classroom machine.  Instead, you can save them to your school's OneDrive account, where they are stored on the cloud and accessible across multiple devices.  For more information on OneDrive, please see the following link: What is Microsoft OneDrive and how do I use it?