Cassidy, previously known as Lance, said she knew she was a girl from a young age. She'd gravitated toward Barbie dolls, lipstick and dresses and was excluded by male classmates for years because of it.
In middle school Cassidy told classmates she was gay to try to blend in. It wasn't until her sophomore year that she publicly dressed as a girl — on Halloween.
This year, as a senior, she came to school as herself.
The school's staff encouraged her and she received mostly positive responses from students.
On Friday, her classmates chanted her name as she walked over to accept her prize in a green and pale pink gown.
"I'm speechless. I can't even believe this," Cassidy said. "I'm so proud of my school, my administration and the student body for making this happen."
Cassidy said she wants to become more involved in the lesbian, gay, bisexual and transgender community. And she hopes her victory inspires other teens struggling with their identities.
"It doesn't matter what people say about you, and the intolerance you may face doesn't matter," she said. "You can get through all of it, and you can thrive through all of it."
Information from: The Orange County Register, http://www.ocregister.com