A 12-year-old boy from England, who showed up to school as a girl, is believed to be the youngest person in the country preparing to undergo a sex change operation.

The boy, whose parents have legally changed his name to a girl's, arrived to school in a dress with long hair.

Teachers called an emergency assembly to order fellow students to treat him as female, but angry parents said that was not enough.

They said the school should have informed them in advance so they could have prepared their children for the news.

"Maybe we could have explained sexual politics and encouraged our kids to be more sensitive if we'd had a chance to be involved," one mother said.

The 12-year-old is preparing to undergo hormone treatment and surgery — and could become the world's youngest patient to have a sex change in the coming years.

RELATED: An interview with the world's leading sex change surgeon.

The Sun knows his identity but will not reveal it, in order to protect the child's privacy.

"We are committed to ensuring the very best for our child," the boy’s mother told the newspaper Thursday night. "We are working with other agencies to ensure our child's welfare is protected."

The 1,000-student school, in southern England, has given the boy a separate bathroom and changing room.

Earlier this year, Kim Petras from Cologne, Germany, who was born Tim, became the world’s youngest person to undergo sexual reassignment surgery. She had the operation just days after turning 16.

Click here to read more from The Sun.