Hello,I am the person that will be helping you today.
May I ask for the full model names of your HP Chromebooks?
I will be standing by, looking forward to your response.
Have you already tried resetting settings on the Chromebooks?
You cannot see the screen at all? I was under the assumption that you could see it a little bit. Is the screen just dark or is it non-functional?
If you turn the brightness down, does the screen get even darker?
When you press F6 or F7, you don't see any change whatsoever in the screen brightness?
Thank you for the update. Often times, this can occur if the LCD connector becomes loose. Have you checked this issue on any of the four Chromebooks?
The instructions found on the following page will show you how to get to the LCD cable: https://youtu.be/H-X6d3L6kWs
At the time 3:05 is when they get to the LCD cable connector. At this time, disconnect the LCD cable and reconnect it securely. Once that is complete, put everything back together and test it once more.