This is why school uniforms are a good idea.
It would happen with far more frequency during the political season if uniforms weren't mandatory. 
*and in regards to what's worn, most things are. And in regards to how teachers can comment on what's worn, freaking out like this is certainly not.
I'm not suggesting that wearing political gear would become standard but there's definitely a greater chance of someone sporting campaign attire when "regular" clothes can be worn every day as opposed to the occasional dress down day.
I wore t-shirts such as this one
and this one regularly to school. That's a lot more objectionable (in a straightforward, mainstream media sense. In another sense, perhaps no) than a t shirt for a mainstream election candidate.
No teacher has a right to get angry about it. Of course that was back in the nineties. There was a story recently about a female teacher here reacting somewhat similar, if not *quite* as badly, over a 14-year old girl's (apparently
very, on that particular day) skimpy attire - and that brought memories back to what some of the girls in my class sometimes wore on summer days (and of a shocked exclamation by a student, though this must have been at age 16 or 17, directed at another student, "wait, are you actually sitting there topless?" She wasn't, but it was hard to tell from the back.) No one would have considered it appropriate for a teacher to get angry at that back in the 90s either.
Teacher in this new case ended up paying a criminal fine, so all's well. -_-
