POP singer Katy Perry made the teenage dreams of hundreds of year-12 students come true when she crashed Melbourne High School's formal at the Grand Hyatt on Friday night with her Aussie gal pal Ruby Rose.

The dynamic duo were out on the town and decided to make a surprise appearance at the event, slipping in to the hotel's ballroom through a back door around 11pm, just as her hit single California Gurls began to play.

Perry grabbed a microphone and waded through the sea of students on the dancefloor before she began singing along to her track.



The Teenage Dreams singer also joined in on singing Beyonce's Single Ladies, as the students jostled to dance alongside her. Watch our video to see Perry in action.

Not surprisingly, the teenagers went nuts when they realised they had a superstar in their midst.

Perry did not linger long after her song finished, but posed for plenty of pics before dashing out the door with Rose.

Pity the poor person booking the entertainment at MHS's formal next year - Perry is going to be a tough act to top.

"I don't know who's year 12 formal it was but nothing was going to stop @katyperry...,'' Rose wrote on her Twitter page.

Party girl Perry acknowledged her gatecrashing: "I totes just crashed a prom!!!'' she Tweeted.

The pair continued their big night on the town, having a good time at the Spice Market nightclub at the Grand Hyatt and then Boutique nightclub in Prahran.

Perry left Melbourne yesterday and is now in Tokyo.