Most of the private schools have Formals for yr 11 and 12. The state schools tend to just have a yr 12 one. The yr 7s have usually had a disco or something traditionally, but this has been pushed out to a graduation night. The main thing is to set a budget and stick with it.......some girls fly to Melbourne for the day to shop for their dress and some parents are willing to fork out $1000 or more for the dress. Lucky boys just have to hire a suit and look clean on the night! AND then there is the outfit for the afterparty :)
But don't worry, bit of shopping around and you can spend a quarter of this.