You have a good point about the other students making their choice. That's the way it is when you select anyone for any honor by a vote of their peers. You never know what their peers are going to do. The adults have to make up their minds beforehand whether they will accept whatever choice the kids make or not.
In the case of electing a transgirl homecoming queen, that was clearly done as a prank, much like in previous years when an unpopular and often ignored or bullied girl would be nominated as a joke, and then the other students would vote for her in a pretty mean spirited show of whimsy.
I give you credit for being intelligent enough to know that the students did not vote for the trannie because they truly thought "she" was the most beautiful and worthy girl in school. Correct me if I overestimate you, please.
It may have been just a joke, or it may have been a statement about the foolishness of the adult-driven transgenderization fad.