Like many young people, Frank and Sarah’s relationship began at a party.

Less usual was the fact that Sarah was an engaged 25-year-old teacher, while Frank was her 17-year-old pupil. Frank, now 29, has no regrets about getting involved, although he recognises that it could be a different story if the school had discovered their secret or if they’d had strong feelings for each other.

“It felt pretty exciting,” he says. “I think we both regarded it as a bit of fun, made all the more exciting because it was taboo. I suppose I was flattered, but we both knew it would not last the distance.”

The affair, which occurred in a south London school, fizzled out harmlessly over time, but anecdotal evidence suggests that they do not always end so well. In a BBC Radio 4 documentary, Married to Teacher, Lisa, then 16, described how she developed a close relationship with her female house tutor. When she turned 18, the two of them embarked on a destructive lesbian affair. “She was jealous of my friends and we never left the house,” says Lisa. “I told her it was over and she said she would kill herself.”

More than a decade on, Lisa still suffers from depression as a result of the relationship, describing her former lover as “manipulative, powerful and disgusting”.

The topic is freshly explored in the newly released film, Notes On A Scandal, which stars Cate Blanchett as a 41-year-old pottery teacher who seduces her 15-year-old pupil.

So how prevalent are these relationships? Very, if the most recent research is to be believed. Pat Sikes, a senior education lecturer at Sheffield University, conducted interviews with pupils and colleagues over 25 years for a study she published in 2005. It concludes that about 1,500 pupil-teacher relationships develop every year.

In her paper, Scandalous stories and dangerous liaisons: when female pupils and male teachers fall in love, she tells of affairs that began, albeit platonically, when one pupil was only 13, and another when a 17-year-old student had a sexual relationship with her 35-year-old study supervisor.

Dr Sikes herself was a 14-year-old schoolgirl when she met her husband, David, a 22-year-old teacher, in 1970. Despite the mutual attraction, the relationship did not begin until two years later when he left the school.

They are now married with two teenage children.

Stephen Lowe, 59, is also living proof that such romances can blossom. He was a married 31-year-old drama lecturer at Dartington College in Devon when he met Tanya Myers, a 21-year-old student. When the relationship started, Stephen declared it to the college. He was still allowed to teach Tanya, but not to assess her.

“I would have embarked on it even if they had sent me to jail,” he tells The TES Magazine. “You can’t legislate against two adults falling in love just because there is an imbalance of power. If you did that, then you would have to stop all work Christmas parties. There’s a big difference between a teacher who is loco-parentis, and a lecturer who may even be teaching someone older than them.”

Stephen and Tanya are still together after 26 years and have two daughters.

“She’s the love of my life,” he says.

But the law has changed, especially for teachers. While most higher education establishments now have strict behavioural policies about student-lecturer relationships, a teacher could face criminal charges. If a professional in a “position of trust” has sexual relations with any of their charges under 18, they could now face prison under the Sexual Offences (Amendment) Act, 2000.

Many teachers have already fallen foul of the law, including Shelley White, 25, who was found guilty in 2005 of “snogging” a 15-year-old boy in her class and placed on the sex offenders’ register.

Meanwhile, Nicola Prentice, a 25-year-old English teacher, was given a 12-month suspended sentence the same year after having sex with a 16-year-old pupil.

The fact that teachers can be criminalised for having a relationship with a 16-year-old pupil at their school, but not with a 16-year-old from another school is “a bit of a nonsense”, according to Chris Keates, general secretary of NASUWT, the teachers’ union.

“I don’t condone these relationships - they are a breach of trust and represent professional misconduct - but they should be a disciplinary issue when it comes to over-16s.”

In the eyes of the law, a 22-year-old teacher having a relationship with a sixth former is just as guilty as the same teacher having an affair with a Year 7 pupil. The NSPCC insists, however, that the Act should be rigorously upheld.

“Young people of school age can be emotionally vulnerable and could develop feelings for their teacher,” explains Emily Arkell, the NSPCC policy advisor. “If a teacher enters into a personal relationship with a pupil, they would be taking advantage of this and abusing their unique position of trust.”

But in Debbie Epstein and Richard Johnson’s book, Schooling Sexualities, the authors explore how some pupils refer to teachers with “an intensity of feeling normally reserved for one’s most significant relationships”. It argues that exciting, thrilling and passionate teaching can create an “erotic charge” in the classroom - but on a psychic rather than a physical level.

“My gut reaction is that pupil-teacher relationships should not occur, because of the inherent power differential,” says Professor Epstein, from the Cardiff School of Social Sciences. “Inevitably, the loser is always the less powerful person: the pupil. But there can be a charge between pupils and their teachers -such as the one (albeit misused) between Miss Jean Brodie and her girls - where it is exciting and pupils are desperate to please. The best teaching has that kind of charge in the context of the lesson.”

However, in between the “psychic” and the “physical” are many blurred stages that can be confusing for teachers. Some, perhaps unconsciously, use flirting as an integral part of their teaching.

Kate Myers, author of Teachers Behaving Badly? was one of them. “I’m sure I flirted with some of the 16-year-old boys I used to teach in the late 1960s,” she admits. “I wouldn’t do it now, but at the time I was this small female teacher, and I wanted to find a way of controlling them without using the cane. I think a lot of teachers flirt with their pupils without even knowing it.”

A sexual relationship, though, is another matter. “It is always the wrong thing to do because teachers have a duty of care whatever the age difference,” she says. “It can never be an equal relationship - the power imbalance inevitably gets in the way.

“My advice to teachers is always to wait.”

TEACHERS IN TROUBLE

Mary Kay Letourneau (left) hit the headlines when she embarked on an affair with her student, Vili Fualaau (below left), in the 1990s. She was 34 at the time, he was just 12-years-old.

The teacher, from Seattle, was married with four children when the affair began in 1996, but she was already pregnant with Vili’s child when she was arrested the following year. She was convicted of rape and jailed for seven and a half years.

A month after she had served her truncated sentence, the pair were caught having sex in her car and she pleaded guilty to two charges of child rape.

She gave birth to their second daughter in prison. Despite the odds, the pair got married in 2005.

State authorities in America receive nearly three million reports of sexual activity between teachers and pupils each year. Up to 40 per cent of the teachers are female.

Chris Woodhead (right), former chief inspector, almost wins the prize for conducting Britain’s most notorious pupil-teacher relationship. He admitted in 1999 that he had a relationship with a former pupil years before, and suggested that such experiences could be “educative”. It sparked a national outcry, but a Government inquiry decided he had no case to answer.

Amy Gehring (right), a Canadian biology teacher working in Surrey, was accused of having sex with three students under the age of 16. She denied four other counts of indecent assault and was acquitted in 2002, but banned from teaching in Britain and Canada.

CHARGED WITH SEDUCING YOUNG GIRLS

Steven Edwards, a married science teacher from Bridlington, East Yorkshire, is currently in court charged with seducing four girls, aged between 14 and 16, at his school.

One girl reported “feeling used” after she lost her virginity to him in his car. Another “besotted” girl did not want to return to school after he started “ignoring” her. At the time of going to press, the case was continuing at Hull crown court.

IF A PUPIL HAS A CRUSH ON YOU

Be aware that an innocent crush can lead to malicious allegations.

Discourage the pupil and ensure that you are never alone with them.

Tell your line manager as soon as possible.

Keep any notes or emails from the pupil as evidence.

Remember that your job - and possibly your liberty - are on the line if you develop the relationship in any way.