How do I reset my iPad to its original settings after using it as a school tablet?

This explains … assuming you (and not the school) own your own personal iPad.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Devices that are configuration locked by an MDM platform, as is common when managed by an Enterprise or Educational establishment, can usually only be released from central management by the platform that manages the device. Be aware that Apple will not assist with removal of management profiles.

Unless you are lucky - and have the option to delete the profile from iPad Settings - you’ll need to seek assistance directly from the System Administrator who manages the device. Otherwise, the device is useless.

Install or remove configuration profiles on iPad - Apple Support