IT IS the event that takes a year of planning and costs thousands of dollars - but there is no wedding dress or cake in sight.

School formals are now rivalling weddings in terms of preparation and cost and have even spawned their own "formal expo".

Australia's first School Formal Expo will be held this month in Sydney, giving students a glimpse of what's in and what's not.

Organiser Graham Stroud said the school formal industry was worth $170 million annually in New South Wales alone as students splashed out big bucks to celebrate the end of school.

The two-day expo will draw on experts in fashion, hair and make-up while providing style workshops.

Kirrawee High Year 11 student Ashley Kent, 16, spent almost $1000 on her Year 10 formal and is already planning what she'll wear for Year 12, even though it's 18 months away. Her mother Shayne said it's ridiculous.

"We spent less than most girls, some girls' dresses alone were $1000. It's like I'm planning my daughter's wedding," she said.

Aquinas College Year 12 student Paige Walker, 17, said planning the night takes all year.

"I've been having hair and make-up trials. My local beauty salon does a package of nails, make-up, hair and a spray tan for about $180," she said.

Year-long planning isn't reserved for the girls - the boys are in on it too.

"I want to look funky, I'm thinking about a cool pair of sneakers with suit pants," Oak Hill College student Thomas Anastopoulos, 15, said.