When envisioning the prom of her dreams, Libby Gonzales, a 13-year-old Texan who is helping plan the first-ever transgender youth prom at the U.S. Capitol in Washington, D.C., had a lot of big ideas.

In many high schools, the pressure is off when it comes to finding that special someone to take to the prom. Instead, teens are going to the prom in groups so big that they sometimes arrive in buses ...