The senior at Marina High School in Huntington Beach was born male but told The Times she always felt like a girl. In high school, she began taking hormone blockers and estrogen injections prescribed by an endocrinologist and made the transition to living as a girl.
She decided to run for homecoming queen, in part, to make a statement.
"If I win it would mean that the school recognizes me as the gender I always felt I was," Cassidy, 16, told The Times before the election.
"But with all the attention, I realized it's bigger than me," she said. "I'm doing this for the kids who can't be themselves."
On Friday, at Marina High's homecoming pep rally, the field of 10 homecoming queen candidates was thinned to five -- Cassidy among them.
At halftime, school district spokesman Tom Delapp said, the five finalists were called up and balloons were released from a box to announce the winner -- white for the princesses and bright yellow and blue for the queen.
"She was stunned. She kind of broke down on the podium," Delapp said of Cassidy. "She was shocked. She cried a lot."
Students started chanting her name and then ran down to give her a hug, he said.
"I was so proud to win, not just for me but for everyone out there," Cassidy told reporters afterward. "I think it really shows the progression of the times."
Los Angeles Times