So do not sign into his school account? That sounds like they are requiring an MDM be installed on all devices used to access school resources. This is standard

So do not sign into his school account? That sounds like they are requiring an MDM be installed on all devices used to access school resources. This is standard security practice. When you do a full reset (there are multiple types on Chromebooks) it should remove everything. If you sign back into that school account, however, it will re-add it. If you create another account (not your school) then it should work as normal. You would need to contact the IT team at your school to find out if they setup this as a full device MDM (and if your chromebook is enrolled or not) or if it is just MAM mode and monitors things when using the school account.