A Seattle teacher is to marry an ex-pupil she had two children by, less than a year after serving seven-and-a-half years in prison for sexually assaulting him.

Mary Kay Letourneau, 43, a registered sex offender, and Vili Fualaau, 22, will tie the knot on April 16.

Ms Letourneau started an illicit sexual affair with Mr Fualaau when he was 12, and a pupil in her class at Shorewood elementary school in suburban Seattle, and she was a married mother-of-four, aged 33.

She was expecting their first child when she was arrested in 1997 and convicted of second-degree child rape.

Ms Letourneau spent 80 days in jail, but was re-arrested soon after her release when the two were discovered having sex in a car, and sentenced to seven-and-a-half years. Their second child was born behind bars.

Ms Letourneau spent nine months in solitary confinement for trying to contact Mr Fualaau from prison. Their children have been brought up by his mother. Ms Letourneau’s former husband has custody of their four children from their marriage.

A judge agreed to remove an injunction against Ms Letourneau seeing Mr Fualaau on her release last August.

Mr Fualaau dropped out of secondary school but is now studying for his school-leaving diploma and is an aspiring artist.

The case has captivated US tabloids and Ms Letourneau has had a TV movie made about her life.

Female sex offenders are much rarer than males - representing between one and 15 per cent of perpetrators - but can cause just as much damage, said Kathleen Faller, professor of social work at the University of Michigan, who works with abuse victims.

“Boy victims of females often don’t perceive it as harmful - they see it as an opportunity,” said Professor Faller. “But there’s still an impact in adulthood. They don’t see boundaries most people see in terms of appropriate sexual contact.”

Despite their rarity, US schools have been rocked by a spate of recent high-profile cases involving allegations of female teachers molesting under-age male pupils.

Last week, Tennessee physical education teacher Pamela Rogers Turner, 27, was charged with statutory rape for allegedly having sex with a 13-year-old boy.

In Texas, English teacher Kathy Denise White, 39, was taken into custody for an alleged tryst with a 17-year-old student.

Florida reading teacher Debra LaFave, 24, is tipped to plead insanity for allegedly having sex with a 14-year-old pupil in her classroom, car and home.

In Los Angeles, middle-school teacher Sarah Bench-Salorio, 28, faces charges of having sex with three students, 14 and younger, from her English class.

And Silicon Valley teacher Rebecca Boicelli, 33, was arrested last month after paternity tests found a student, who was 16 at the time, fathered her child.