''Someone came over to me and said Katy Perry had heard about our formal, and wanted to come,'' says Lay, 17. ''But she's a celebrity and there were more than 550 students in the room, so another person, a woman, said it would be too dangerous.''

But Katy Perry is persistent when it comes to quirky publicity stunts. At 11pm, one of her entourage approached Lay and asked him to talk to the lady herself in the corridor.

Click on the gallery at right for more photos

''She said we could just walk in together,'' says Lay, who agreed to accompany the pop star, soon to marry comedian Russell Brand, across the room. While ''10 or so people started following us right away'', says the still-dazed year 12 student, the crowds didn't realise their party had been gatecrashed by a celebrity and her entourage - including MTV presenter Ruby Rose - until Perry grabbed a microphone and walked into the centre of the dance floor.

''Well, this is boring,'' she said, and started singing along with Beyonce's All The Single Ladies. The DJ switched to Perry's song California Gurls and the students started swarming.

''I saw everyone rush the dance floor. I thought maybe it was just a good song, but then everyone started screaming her name so I ran down,'' says year 12 student Stefan Uzelac (a member of Victoria's under-20 basketball team). ''It was a pretty compact circle around her. Everyone was engulfing her, taking photos, screaming.''

Perry was on twitter by 11.45pm, broadcasting her stunt: ''I totes just crashed a prom!''

Ruby Rose posted photos of the thronging fans yesterday morning: ''I don't know who's [sic] year 12 formal it was but nothing was going to stop @katyperry …''

Two days later, Lay is delighted that a school better known for its scholarly alumni has become the coolest of them all. ''It will certainly bring up the school's party reputation, no school will ever beat that.''