Do you get that error if you try using a different account (non-MDM, for example a personal Gmail account) after a factory reset, never using* the MDM account post-reset?

* Feel free to use it in a third party browser such as Firefox or Brave, as another commenter suggested. Just don't use it for an OS login.

Ask them for an administrative hearing. 99% chance they won't and will just remove it.

From your description it sounds like when your child signed in with the student account, one of the Ok's they hit for GoGuardian put your device on the schools managed account(cps.edu).

If you're powerwashing and it's automatically pulling cps.edu managed policies as soon as it gets online then the school district IT needs to release your device, and then powerwash it and setup again without the cps.edu policies.

Be aware if you want your child to do any official testing or use their student accounts on your personal device the district may require this arrangement.