Katy Perry shocked a group of Australian high school students!

Katy went to the Melbourne High School prom at the Grand Hyatt, which is the same hotel where she was staying during her tour down under.  Towards the end of the night, everyone was having a good time, when Katy walked in and crash the party!  It turns out Katy was walking through a back door of a function room when she heard "California Gurls" playing. Katy grabbed a microphone and started singing along to her track.  Witnesses said she was slightly drunk and was stumbling a little. She danced and laughed with students until the end of the song, then left with her security guards.  Katy later Tweeted: "I totes just crashed a prom!"