How to get rid of supervision
The serial number may be claimed - correctly or incorrectly, you have two options.

If you can get to settings see if you can determine the exact address of the management server (get screenshots) and then try to remove the profiles.
If that doesn’t work, you have to handle this administratively with the School and/or Apple. Collect all your receipts for purchase so you can help clarify how you came into possession of a device that’s marked as school owned.