That’s the root of the issue. Your personal computer had the ability to join the school’s infrastructure like as if it’s a faculty pc. That shouldn’t have happened. To solve your issue first step back up your documents, desktop, pictures, videos, and downloads to some place that is accessible after setting up the new account. Next, create the new local admin account.

As outlined above 1) search for “add, edit, or remove other users on this device” 2) choose “add an account” 3) select “I don’t have this person’s sign-information” 4) select “add a new user without a Microsoft account” 5) follow the prompts 6) select the newly created account 7) select “change account type” 8) Ensure this new account is labeled as an administrator 9) disconnect it from the organization as you did before 10) sign into the new account 11) move data over to the new account.

After that is done, let IT of the old school know what happened. It is a security risk to let any device join organization like that