My school had 'celebration day' on Wednesday; the theme was 'fantasy' so we had all kinds of crazy characters.
I'm guessing you go to one of the better schools in Melbourne, because my school would've gone with a brothel-themed celebration.
Oh, it's been done every year. Any costume idea has become a 'slutty x', in the same way that anything halloween can be 'zombie x'
School buildings were locked, anything we did to the yard we had to clean up. No pranks happened. Everything was pretty mellow.
Most people went into the city afterwards, and confused the hell out of all the people around. Because I go to an all girls school, some people thought it was promotional for a product or a company or something :/ . Other people thought we had dressed up just to see the queen. <__<'' Some even got on tv.
Then grad night afterwards.