The warnings to new members of staff were always the same: never put your hands on a pupil; do not offer them lifts; do not share your mobile or home phone number; do not text or email them; take care with social networking sites and remember – it is against the law to abuse a position of trust.

Teachers and classroom assistants at Headlands school were told when they began that it was "a criminal offence to have a sexual relationship with a pupil under 18 regardless of consent", and the message was repeated in sessions every September, it has emerged.

Yet it was not enough to stop the Yorkshire comprehensive being branded a "school for scandal" when Christopher Reen became the fifth member of staff in just three years to face criminal charges over relationships with pupils.

The leader of one of the country's largest teaching unions said yesterday that she was staggered one school had faced so many cases. "It is absolutely extraordinary and I am sure the local authority and school management will want to look closely at the processes they have in place," said Chris Keates, general secretary of the NASUWT.

"It is extremely rare for teachers to actually cross that boundary. We get around 800 allegations a year against members and the vast majority are about physical abuse of pupils. A very small number, maybe half a dozen, are about inappropriate sexual conduct with a pupil. Even fewer go to court and not all those end in a conviction. It is a tiny number of teachers – so to have such a concentration in one school makes that an extraordinary situation."

Reen, 31, faces jail after admitting having a sexual relationship with a pupil to whom he sent hundreds of text messages. But the classroom supervisor – who had recently married and whose wife was pregnant – claimed he did not know the girl was under 16. A jury was discharged last week after it failed to reach a majority verdict over whether Reen was aware of the girl's age. He would have faced a much longer sentence had that been the case. The Crown Prosecution Service was given seven days to consider a retrial.

But what makes Headlands extraordinary are the cases preceding Reen's. The first scandal to engulf the school was that of Ian Blott, 55, an art teacher who was jailed for four years in 2006 after seducing a 15-year-old girl. Just a year later, science teacher Steven Edwards, 35, was given a sentence of four years and nine months after being convinced of having three affairs with teenage girls. Then in 2008 Terry Mann, a former Headlands IT teacher, received a 12-month suspended sentence for having sex with an under age girl at another school. In the fourth case, a female classroom supervisor was given a conditional discharge over her relationship with a boy.

"We expected parent groups to yell about it," said one local resident who attended town council meetings. "Every week people talk about parking and shopping and other issues – but no one ever brought up this. Of course we knew it was unusual to have so many cases in one school."

In fact, the issue of Headlands was raised once at a town council meeting in Bridlington, East Riding, this summer, but officials decided it was for the school and the local authority to deal with.

East Riding council, meanwhile, insisted that it had tried to stop the rot. Training sessions for all new staff covered appropriate behaviour with pupils, while governors and senior staff were given special sessions on recruitment and dealing with allegations. By next term all staff at Headlands will have completed a module on "Awareness of child abuse and neglect". Regular warnings reminded staff to avoid being alone in a room with individual pupils and to tell child protection co-ordinators if a pupil's behaviour was causing concern.

The executive headteacher, Chris Abbott, said the school tried desperately to stop further scandals and brought in a "whistleblowing policy". She said: "This highlights that while the school has done all within its power to prevent this happening, we still need the vigilance of staff, pupils, parents and residents to highlight any concerns they may have. Safeguarding children is the responsibility of us all."

In Bridlington, many people say they remain supportive of the school – saying the blame lies solely with the individuals involved. Nevertheless, the number of cases has astonished many residents who point out that the town's other secondary school has had no such trouble. "I would like to think that lessons have been learned and that this would not happen now to any other family and that there's an end to it," said one parent. "I think after five cases, surely systems must now be in place and people should know the severity of it."

East Riding is not the only council to drill all new teachers about how to behave appropriately with pupils – it is something that is spreading across the country. The General Teaching Council for England runs courses for trainee teachers and addressed 10,000 of them between March and July this year.

"We have a session that we provocatively bill: 'Is it anybody's business what you do in your private life?," said Fiona Johnson, director of communications at the GTC. "We talk to them about what it means to be a professional and one thing some say is: 'I guess I will have to leave Facebook behind'.

"It is not so much that they can't use it but it is more about being alert to the idea that they may need to ring-fence who can access their profile."

Teaching unions also offer training. Keates described how the NASUWT spoke to newly qualified teachers about the risks they could face. "We talk to them about their vulnerability to young people developing crushes on them and wanting to develop too close relationships," she said. "We tell them that they need to be very alert if a pupil becomes overly attentive – coming into the classroom after school finishes or hanging around their car – and to tell a senior member of staff. It is a very difficult thing to deal with and teachers can find themselves victims of false allegations because the pupil feels slighted."

Keates said the union had also approached the Training and Development Agency for Schools encouraging it to include clear messages in teacher training. "But most people know the type of relationships they should develop with young people and the types they should not," she added. "The problem is the culture that has developed in schools – teachers are finding themselves in more difficulties because they are encouraged to get very involved with pupils."

There was an expectation for teachers to be "more accessible in their attitude", she said, and more available out of school hours and by email.

In one case she described how a teacher spoke to pupils about poetry via a webcam in his home. Technology, added Keates, could be dangerous because email messages were less formal and more open to misinterpretation. "Our concern is that this new culture is starting to break down the boundaries of professional relationships and make everything more informal – and that starts to muddy that clear distance that has to be kept.

"It doesn't have to be a threatening distance, but in all communications there should be no chance of it being misinterpreted."

She added: "The changing culture in schools, the increasing use of technology, the additional contact outside the classroom – that might explain some of the things that have happened but it doesn't excuse them."