ChromeTools requires student devices to be on Chrome OS 59 (January 2017) or above.
You can check the version on the Chromebook from Settings, Left Menu, About Chrome OS.
You can check the version of a device in Control Tower by going to Devices > Chrome Devices and finding the device in the list. Note that you can sort the table by OS version to pull all the old ones to the top. Those that are more than 5 versions out of date will be highlighted in red.