You have several free cloud based options: iCloud, Dropbox, and Google Drive - just to name a few. You may even want to purchase a large usb flash drive or an external hard drive specifically for this purpose. If you want to get serious you can even use Time Machine. The important thing is - Just Do It!
Just in case... this video can walk you through creating a basic back-up.
Please help SPREAD the news! Tell ALL your students how important this is!