Using Ctrl+Alt+PrintScreen will copy (to the clipboard) the top layer of what is currently showing on your desktop as an image. You can then paste it into a document and crop or edit the image as needed.
Ctrl+ "switch window" captures entire screen. Ctrl+Shift+ "switch window" lets you select a range to snip. Of course, any time you're on Chrome or on a Chromebook, you can also use extensions like Awesome Screenshot!
When all else fails, don't forget that you (and your students) can take pictures with your phone! Email them to yourself or upload directly into Drive or Classroom.