The Spring Fling Queen Tiara
Pop quiz: who's the last person you saw wearing a crown? Maybe you watched The Princess Diaries for the 53rd time on cable the other day. Maybe it was Queen Elizabeth II.
Crowns and tiaras symbolize superiority. They're worn by the ruling class.
The Spring Fling Queen tiara symbolizes superiority through popularity. Cady didn't get those votes because she's good at calculus; she got them because she's really popular—in part because people think she shoved Regina in front of a bus. Cady being crowned Spring Fling Queen is the completion of her journey to usurp Regina.
She's not just the Spring Fling Queen; she's the queen bee.
When Cady then breaks the tiara—to audible gasps, no less—she's rejecting all that the sparkly headgear represents and making her own symbolic gesture. She's pointing out how silly this whole Spring Fling thing is, from ballot boxes to expensive dresses to cheap plastic crowns. The tiara is just one more way to create class warfare between the social strata at North Shore, and Cady's over it.
As she redistributes the tiara piece by piece, she says:
"I think everybody looks like royalty tonight."
All it took for her to learn that was a violent brawl between the junior girls, Regina getting hit by a bus, police searching Ms. Norbury's home, failing calculus, barfing on her crush, and completely alienating her mother.
So yeah, some lessons are harder to learn than others. The important thing is that we learn them.