Constance McMillen wanted to go to her high school prom. Like most students in the United States, she doubtless saw the event as the capstone of four years of effort, a gala event for students that represents a reward for getting to the end of their senior year and, presumably, graduating not only from high school but into adulthood. One night of glamor and revelry, dressed at a level of style and affluence many might never indulge again, to celebrate the matriculation into the next level of independence. A party where students can show themselves—to their peers and to themselves—as adults.…