We have noticed that the main reason that keys are falling off of this model of Chromebook occurs when students are putting the Chromebooks away in the black Ergotron carts. This usually occurs if the student attempts to put the Chromebook in the slot on the cart, but accidentally gets the plastic divider between the keyboard and the screen. The result is the divider slides across the keyboard and any keys that aren't perfectly flush get popped off from the force.
We know this is the cause as the same model of Chromebooks were put into some of our newer carts with the rounded edge dividers and those have not had any issues with the keys. It doesn't help that the Lenovo models also are designed with the edges rounded in, making it even easier to accidentally insert between the dividers.
To prevent this from happening and to reduce how many Chromebooks are damaged, students must carefully put the Chromebooks away and ensure that they are not getting the divider in between the screen and keyboard. It may also help to put the Chromebooks away with two hands, one holding the lid closed so that it can't be separated by the divider.