tldr, you have to disassemble the chromebook, and use a flashprogramer.
also, please don't do this with a stolen device.
I get that schools give these away without properly releasing them.
only worth it if you like a challenge, and have the time.
expect to brick the device at least once, so long as you don't blowup the bios ic and you have a dump of the bios chip, you can recover it.
if you want to run linux you will need to flash coreboot on it using this project mrchromebox.tech
the bios chip will need a manual flash using a ch431a with a 5v to 3.3v converter, or a ch431a modded to run at 3.3v.
see this page for instructions on using the programmer
https://wiki.mrchromebox.tech/Unbricking
(heads up, I had some issues with the flashrom app in the manjaro repo. ended up using the other flashrom app in the repo "flashrom-starlabs" and that worked)
after finding out which device you have using this chart
https://mrchromebox.tech/#devices
you will need to manually download the needed coreboot file, you can put together the download link you need from here
https://github.com/MrChromebox/scripts/blob/master/sources.sh
once you have the needed coreboot file, you will use a dump of the locked chromebooks bios chip and extract and inject its (serial, eth mac address) info into this new coreboot file, then you will flash this modded file with flashrom and the ch431a
if you want to run chrome os, you will need to change the serial number, I've heard using all zeros works, but haven't tested this (I use linux)
changing the serial number can be done by using another chromebook with dev mode active and access to the terminal.
you will also need a dump of the bios chip, use a ch431a with a 3.3v converter.
then you will need a program that comes with the dev mode chromebook that can change data in the dump, then you need to flash this modded file back onto your locked down chromebook.
the app you need is called vpd or something, you might find it in a repo somewhere (I didn't look very hard since I didn't really need it).