There's not much troubleshooting for the Chromebooks, but these things can solve quite a few problems.
1. Perform a Hard Reset
Hold down the power button for 4 seconds to force the device to shut down. Plug in the device to a charger. While holding down the refresh button, tap the power button on the left side of the device.
2. Remove and Re-Add the User
Have the student sign out of their device so that they're on the screen with their name. Click the down arrow next to their name and remove that account. This will take them to the main Google sign-in screen where they can login again.
3. Check the Accessibility and Keyboard Settings
This one you may have to try from both the sign-in page (device settings) and after the student logs in (user settings). Either one can cause problems. Click on the time in the lower right corner and a list of available settings will pop up. Common ones that can cause problems are screen magnifier (Accessibility settings) and keyboard layout (next to accessibility).