The firmware is locked down with typically either a screw (older devices) or a CR50 security chip (newer devices): https://wiki.mrchromebox.tech/Firmware_Write_Protect#How_Does_Firmware_Write_Protect_Work.3F
The problems with loading a different distro on them would be:
- Cost to go through the process of installing alternate firmware and a new distro on hundreds of devices
- Cost to setup an alternate system to manage/track the devices
- Cost to deal with students who can now more easily re-flash the devices to run other things
- Loss of the fairly extensive management capabilities that ChromeOS provides that allows a school/government to lock the devices down, monitor them, etc