I think the court will rule in favor of the girl.
The shirts do not have an obviously negative image. Instead, they are actually positive. Anyone making the connection to the principal either knows the story, or was told the story, so the disruption remains the fault of others, specifically the principal.
Clearly his actions continue to impact his credibility, which, to me, raises serious questions regarding his fitness.
You make a good case. I think the girl could have gotten away with it if she'd left it at just the first shirt.
The coincidence of the 2nd shirt's acronym spelling DUI, though, was a bit too cute. I think the court will see through it.
There's also some reason an 18 year old is just a junior at this High School. I'm betting that these two have a history, not reported in this article, that will also come out.