can’t software update because of parent password

If what I fear the most.

There is Nothing More you can do On You Own - Full Stop

It appears from your description

The School Has Locked you Out of Erasing the Computer

Even if you were above to Erase the computer

The Serial Number that is encoded on the Logicboard of this computer is Registered to and by the school in their MDM Service

Meaning, at any time that can once Again Lock the Computer

You only choice I see, bring the computer to the High School and have them Release this computer to you - full stop

This all sound like the High School may nave enrolled this computer in their High Schools' MDM Service

Intro to mobile device management profiles

You would need to Remove this Profile ( if it can been done by you )

Or seek assistance from the High School I.T. Department

You are welcome

Did you purchase this computer with your Own Money ?

If not, only you know that answer

The School could place this computer in Lost Mode rendering this computer Total Useless even for Trade In purposes