Maybe I’m just too paranoid after having been burned one too many times, but I never ever depend on a Save happening automatically even if that is the supposedly advertised behavior. Save early and save often is the way to go. You also never know when your browser might crash or your internet connection goes south. You also probably want to save local copies on your own computer by doing “File → Download as notebook (.ipynb)” at least occasionally. Websites have been known to lose files. If you want to save all the files associated with a given assignment, here’s a thread about how to do that.