IT trouble comes in many forms but there a regular set of techniques to help when students can't get what they need for their classes.
Try these in order:
Restart the device - Always a good first move. Fixed a remarkable number of things
Confirm they are logged in with the correct account - some students are able to see a document/file but not open it. Also see Google Issues
If using and iPad, Uninstall Google Drive or Classroom and Reinstall the app (any other apps too) - this has resolved issues for a LARGE number of students today.
Switch to using a browser - using a browser instead of the app can clear up if it is a cache issue.
Student home network issues - many times, the student's home network or device is just not strong enough to accomplish certain tasks. If none of the above work, please have them send in a help desk ticket at firstname.lastname@example.org