Ya know, this is just not a hill worth dying on, in my opinion.
I would write a letter of apology.
BUT, in it, I would mention freedom of speech.
In the grand scheme, this is a small hurdle.
No, I don’t believe they have the right to keep the diploma, but really, with so much else going on, college to start soon, etc., I’d just write the letter and be done with it.
This is where I disagree.
I don’t respond well to bullies, not well at all. And, this is nothing more than a bully tactic.
If your boss made you “perform” to get your paycheck, would you tolerate that? Want the keys to the new car you just bought ... well, first you have to do something to “my satisfaction” ... and if not, well ... I keep the keys until I think you deserve to get them. .... wrong!!
School is an institution for learning, and this teaches the opposite lesson that should be taught. This is nothing more than a power play by people who feel that they have the right to make demands upon others - and my view is to knock them down so far that they will be begging forgiveness - and will never be placed in a position of authority again.