Unless it was school provided hardware, management is done at the account level and accounts are fully isolated.
Powerwash (https://support.google.com/chromebook/answer/183084?hl=en) the device, login with a personal account to make it the primary, and then login with your kids school account. Yes they will be monitored when logged in to the school account, but that is for compliance with the law. If you don't like it, write your representative.