If your users do not see a preview of their student screens, ensure that Enable screeenshots is selected in the Google Admin Console.
Follow these steps to enable screenshots in the Google Admin console:
Login to Google’s Admin Console with your organization’s admin credentials:
Click the Device management icon.
Click Chrome management from the list of Device Settings (left-side menu).
Click User settings.
Choose your organization from the Organization list.
Eneter screenshot in the Search settings field.
Choose Enable screenshot from the Screenshot drop-menu.
Click Save to apply the new setting.