Pretty dresses, smart tuxedos, loud music, big silver disco ball and non-alcoholic punch.
The formal dance held for Melbourne High School in the Grand Hyatt Hotel was going to plan.
But things were going to get ever better.
Katy Perry, who has been on tour Down Under, heard her hit single California Girls playing in one of the ballrooms as she returned to her hotel room.
The 25-year-old's curiosity got the better of her and she impulsively wandered in to investigate.
On realising that she had crashed a high school formal dance, she delightedly grabbed a microphone and sang the rest of her song, much to the amusement of the guests.
She also treated the crowd to her very own version of Beyonce's Single Ladies, complete with dance moves, before being hurried back to her hotel room by her minders.
"Everyone went nuts and it was the perfect ending to our evening," one party-goer said.
See more celebs who put their fans first.
Photo: ę Getty
Previous News In Pictures: