It can’t be done. The way Google does management it makes it impossible. You need to change the S/N and reset the Chromebook to remove it, but if you ever default back to the correct S/N you also bring the management back.
Note: The following two sections are only for legal and educational purposes – with consent. Since Powerwashing a managed Chromebook will unenroll the device from any managed domain and unlock it from its own network, it is illegal to do the mentioned without proper consent from your school, ...