An Auckland Catholic school asked boys to dress as girls for an upcoming concert to make the occasion more "fun and silly".

But St Peter's College is now reconsidering after a complaint that the idea made light of gender complexity.

One parent said she was shocked after an email was sent home ahead of next week's Eisteddfod, giving costume requirements for a show that sees boys in years 7 and 8 face off in singing, music and poetry over two nights.

The letter, sent by a year 8 teacher, told parents that their intermediate-age boys would need to don feminine attire for the evening's entertainment to make it "silly".

"In regards to costumes, for our song and dance the boys need to be dressed as girls. I have suggested borrowing their sister/cousins clothes," the email said.

"Also, borrowing hair accessories such as headbands or clips to make the night more fun and silly would be awesome!"

The parent said gender complexity was a sensitive, serious subject and something that should not be joked about.

"You would think given recent attitude changes in media and society that a school would be more sensitive to the issue of gender identity and how it may affect our sons, who may be unsure of their own gender identity, or how they interact and think about others who are either unsure or comfortable," said the concerned mum who did not want to be named.

"Dressing up as a girl is certainly not 'silly' these days nor to be made fun of in a show."

The mother said her son had since asked to buy female clothes as it would be "funny".

"I will be doing my own re-educating," she said.

St Peter's College headmaster James Bentley said he understood the parent's concern and agreed it was probably not an appropriate comment to make in the current climate.

"It's very much a throwaway line in an email from a teacher who didn't appear to think about the language used. It's a comment that I'm sure she wishes she hadn't said."

Bentley said it was all light entertainment and fun and to have the boys dressing up in the different characters was not meant to cause offence.

He believed the teacher was inferring the item would be fun.

"I don't think they're trying to mock gender," he said.

However, the teacher who sent the email was reasonably new to the profession and not necessarily aware of the tone of the correspondence.

He agreed that dressing up as a girl wasn't something to be made fun of and changes could be afoot as a result.

"I can see why the parent has come to think that.

"I guess as a boys' school they're playing different characters, some people dress up as trees. Is dressing up as girls now appropriate? Probably not now in this climate. Maybe that's something that needs to be looked at."

Eisteddfod, a Welsh festival of literature, music and performance dates back to the 12th century. The word Eisteddfod is derived from the Welsh word eistedd, meaning "sit".

All intermediate-aged pupils take part in the annual competition which has been part of college tradition for more than 80 years. The boys can participate as a member of a class item, group item or performing a solo item.