I'm not sure that, on this, I'm entirely against the school. They should not have altered the pics.
But I am not sure they should have allowed them to be taken, either.
A yearbook is an inappropriate place to have political material. It's about school and school memories.
If I recall correctly, MY schools DID review pics that were to go in and re-took pics or otherwise blocked the publishing
of pics that said certain things. (My senior yearbook DID however miss ME subtly flipping the bird in a group pic).
Vulgar material - or suggestive T-shirt slogans (two ACTUAL shirts from my school days - a pizza place that prominently said "The best piece" right on the front, and a hot dog place that boasted on its shirts "the best meat between the buns") were not allowed. But that went for gestures (ha!) and even slogans on shirts. (Gang signs weren't a "thing" then).
I get what you're saying, but.... These weren't "#### Trump" or "#### Hillary" shirts, they were just pro-Trump. The quote that was censored, said long before Trump was president, was "If you're going to be thinking anyway, think big." If the quote was "Grab 'em by the pussy, and they let you" that would be pretty inappropriate for a school shirt/quote/picture. But, I'm not even sure of that, because some would argue "Give me liberty or give me death" should be censored because it is "inflammatory" (and rightly so - it IS inflammatory on purpose).
Trump's win was historic in that it is the first time (to my knowledge) someone without government or military service was elected. Whether it was historic or not, he IS the president, and positive/non-controversial statements by him seem more than appropriate; I would say appropriate even for schools, which should be
teaching presidential positions, and asking (without judgement) what students think about them - facilitating open and thoughtful discussion.