Can my highschool withhold or write bad notes on my transcripts post-graduation?

Instead of asking whether the school can withhold supplying your transcripts or make colleges aware of a disciplinary issue, ask yourself whether "senior tag" is worth possibly jeopardizing your further education or career. You seem smart and articulate; I think you'll come to the conclusion that there are far, far better things to do to commemorate graduation instead of that.

Private schools can do darned near anything -- or think they can which is almost the same. In some cases, they rescind the diploma and even contact each entity to whom they have sent out your transcript. Proper? No. Legal? Possibly. Worth the gamble? Not to anyone over the age of 14.

You are in a private school setting and need to be aware that many private schools are able to make their own rules as they see fit. It may not be the "right" thing to do, in your opinion, but it since the private school makes the rules, there would not appear to be any illegality at issue. As the other attorneys have stated, it is not worth risking your bright future in making this issue a controversy.