Affordable fashion: Lifeline Liverpool manager Lisa Hutton with some of the second-hand formal gowns on sale. Photo: Anthony Johnson

School formals are a rite of passage for teenagers at the end of the school year, but as expectations rise about the trimmings involved - a new dress or suit, getting hair and make-up professionally done, car hire - for some the cost is too much to bear.

A teacher at one western Sydney high school contacted by Fairfax Media said that each year about one-third of year 10 students at the school did not attend their formal.

There are teachers who spend their own money, bring in suits, ties or dresses to help a student discreetly. 

The teacher believes this is because even though efforts are made to keep costs down, the students simply cannot afford it.

''They'll say they're not interested, it's boring, and will just go to the party afterwards,'' said the teacher, who asked not to be identified for fear of embarrassing her students. ''There are teachers who spend their own money, bring in suits, ties or dresses to help a student discreetly.

''In the past I've given kids my husband's ties and shirts and we might talk about it amongst ourselves, but we allow the kids to save face.''

The president of the NSW Secondary Principals Council, Lila Mularczyk, who is principal of Merrylands High School, said the costs were becoming prohibitive, especially for girls. She, too, has had students say they were not going because of the cost involved.

''If I think that's the case I will confidentially drag the person into my office and will have a closed-door discussion because I couldn't bear it,'' she said.

Many charity shops do a strong trade in dresses for formals at this time of year.

"I think a lot of the time [students] are looking at the cost of their transport, jewellery and shoes and so they're trying to find something a bit within their budget," said Lisa Hutton, manager of the Lifeline charity shop in Liverpool.

Students can expect to pay $50 to $60 for a 1970s or '80s vintage-style dress at the shop, compared to designs in department stores or boutiques that can cost up to $1000.

The popularity of the op-shop alternative means Ms Hutton will start advertising the store's second-hand formal wear to local high schools next year.

Some teenagers may be embarrassed they cannot afford the expensive suits and gowns their classmates will wear but they have more confidence to browse at a thrift store if they are shopping with friends, said Ellie Schultz, a co-supervisor at the St Mark's Oakhurst Op Shop.

"Some are very upfront saying, 'Well, I can't afford it', Ms Schultz said. ''But usually you find a few that are doing the same thing and they'll come in and do it together … I think some are just shy but you can kind of tell that's what they're meaning: they can't afford it."