Always log out of your computer when you are not using it. There is a simple reason for this: Another student...and one who may prove to be ignorant of the fact they're logged in as someone else, someone who's just mischevious and would like to play a game with you, or straight up mean spirited and wish to undo all of your hard work. Regardless of the intent, you will save yourself a lot of time and trouble by doing this single thing.

Here is a shining example: 

This is "Joe's" folder when he didn't log out of his computer: 

This is how "Joe" found it when he returned the next day: