Dean Dainty was 16 when his teacher Nicola Prentice gave him a mobile phone, telling him it was a reward for doing well in class.

It had one number on it – hers – and her texts quickly turned intimate, and the relationship sexual. She arranged rendezvous between lessons, swearing Dean to secrecy, but after 18 months the strain became too much and he exposed what was going on. Prentice received a suspended prison sentence and was made to sign the sex offenders register; Dean said her actions "took a piece of my life away".

In another school in a different part of the country, a male teacher shared "just one kiss" with a 17-year-old female pupil at an end-of-term party. The teacher was arrested, charged with engaging in sexual activity with a child while in a position of trust and made to sign the sex offenders register, barring him from working with children for 10 years.

Yesterday a row broke out over whether the punishment meted out to teachers involved in relationships with pupils above the age of consent is over-the-top or appropriate.

Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, called for ministers to think again about the law that makes it a criminal offence for a teacher to have sex with a pupil above the age of consent at his or her own school.

But her comments were criticised by child protection groups who say the law – the Sexual Offences Act – is there to protect children from adults who abuse their trust.

The teacher involved in the second case, who insists on remaining anonymous, said: "I'm a convicted sex offender for kissing a 17-year-old girl." The court that convicted him heard he had been engaging in texts and emails with the pupil before the party – one of which had led police to believe he had been involved with "grooming" the pupil for a relationship.

However, he insisted the conviction was "a total over-reaction and symptomatic of the child protection paranoia that now exists", saying that even the judge at his trial described the offence as "very minor". "I'm no Gary Glitter," the teacher said.

Ms Keates said yesterday that putting teachers who found themselves in such circumstances on to the sexual offenders register was an over-reaction. "That's the point where we do take some issue with the law," she said in an interview for the ITV1 Tonight programme, which is devoted to the topic this evening. "This isn't a person who is showing any tendencies for being a sexual offender: this is a person who's made a serious error of professional judgement. I don't think they need to be criminalised by being put on a sex offenders register."

She added: "There is a real anomaly in the law ... that is that if a teacher has a relationship with a pupil at the school at which they teach – it could be an 18-year-old pupil in the sixth-form – then that teacher can be prosecuted and can end up on the sex offenders register."

But Zoe Hilton, policy adviser to the National Society for the Prevention of Cruelty to Children, said: "The law is very clear that if a teacher abuses his or her position by forming a sexual relationship with a pupil they could be prosecuted and this remains the case even if the child gives their consent.

"The law is, quite rightly, there to protect children."

Plans for the law were unveiled by ministers in 1999 to allay fears in the House of Lords that a reduction in the age of consent for gay sex would leave some 16 to 18-year-old boys and girls vulnerable.

Ms Keates said: "Clearly there has to be appropriate disciplinary sanctions in the school to make sure inappropriate relationships don't develop but it does seem a step too far – when there has been a consensual relationship – to put that person on a sex offenders register when, in fact, they could have a perfectly legitimate relationship with an 18-year-old still enrolled at another school.

"I'd be very concerned if it was thought the NASUWT was saying that kind of relationship was OK. We don't think it is. It is gross professional misconduct."

Tonight's programme, entitled "To Sir With Love", also features the story of two other cases where teachers have had affairs with their pupils.

In a separate case, social services launched an investigation into the relationship between a 16-year-old pupil, Jess Anderson, with her thrice-married 50-year-old religious education teacher, Clive Richards, who are now living together at his family home in Penzance, Cornwall.

Jess, who has now left the school, insisted the affair did not start until he had left the school on long-term sick leave as a result of depression. However, when she first moved into his home in January, she was still a pupil at the school.

"Clive and I are in a long-term relationship, we are in love and we are very happy," said Jess who is planning to go to college in September. "We are just a normal couple who happen to have an age gap between us. I wish people would understand that."

Her parents have disowned her after saying she had to choose between them and her former teacher. Social services found there was no case to pursue as she was involved in a consensual relationship and was above the age of consent. The most famous case was that of Chris Woodhead, the former schools inspector, who was said to have had an affair with Amanda Johnston, whom he taught at Gordano school in Bristol when she was a 16-year-old pupil. He always denied the allegation, insisting that their relationship started after she had left the school. Friends of his former wife, including the actor Tony Robinson, said differently. Mr Woodhead went on to have a nine-year relationship with Ms Johnston.

The case received publicity after he was recorded on tape telling a group of trainee teachers that sex with a pupil could be "educative" for both partners.

Ms Keates is worried about the legislation – which has been in force for three years, during which there have been 129 prosecutions – from another standpoint: that it could prompt pupils to make malicious allegations against their teachers.

Figures show that around 6,000 teachers have been accused of abusing children in the past 15 years. The NASUWT said that of the 2,210 allegations it had investigated during this time only 88 had resulted in a conviction.

ITV's Tonight is broadcast at 8pm this evening