Many teachers will be used to fielding such comments as “Sir, Kelly fancies you”, or “Miss, Lewis wants to know if you’ve got a boyfriend”. Faced with hundreds of hormone-fuelled teenagers each day, it’s part of the job description to be the object of students’ affections.

While having your own fan club can be flattering, teachers should tread carefully. Student crushes are often harmless, but a minority can turn dangerous. Handled badly, a crush could cost you your professional reputation - and your job.

Graham Snelling would know. As a young PE teacher, he is often the object of student crushes, but nothing could prepare him for one that almost got out of hand. “When I first started teaching, I was only 22,” he says. “All these pretty young girls were lusting after me. I couldn’t help but feel flattered.

“I’d be out drinking with my mates and students would come up to us. Away from school, all made up, the girls look much older. It didn’t occur to me not to drink in the same pubs or chat to the students when I was out.

“Sally* was a talented netball player and I often drove the team to matches, so I knew her fairly well. Her friends teased her openly about having a crush on me, but I laughed it off.

“Then a member of staff noticed photos of me, stuck to the inside of one of her folders. They’d been taken without my knowledge, mostly around the school. Sally was challenged, appeared embarrassed and handed over the photos to my colleague who advised me to tell my line manager. I thought it was the end of the matter.”

On Valentine’s Day, Mr Snelling arrived at school to find a card and a present for him.

“The gift was expensive,” he says. “The new Arsenal shirt - a big purchase for a teenager with only a Saturday job.”

Again Mr Snelling spoke to his line manager. During a tearful meeting, Sally admitted she had sent the gift. In front of his manager, he explained that he couldn’t accept it or have a relationship with her. Sally seemed to understand and he left the meeting relieved.

A few weeks later, Sally’s parents called the school. They’d discovered a photo in her room, featuring Mr Snelling with his arm around their daughter. When he saw the photo, he realised what had happened. Sally had used a computer design package to cut them out of a group photograph and make them look as if they were alone.

“I remember the photo being taken one night we’d bumped into them at the pub,” says Mr Snelling. ” There were about ten of us in the photo, but it didn’t look like that in this one. Because I’d confided in my line manager, she was able to reassure the head and Sally’s parents that nothing was going on, but I got a ticking off from the head for socialising with students.

“The humiliation was enough to make Sally realise what she was doing and since then, I’ve gone out of town to meet friends.”

According to Susan Quilliam, a relationship psychologist, Mr Snelling’s experiences are not uncommon. “With young people spending the majority of their time in school, teachers often become their role models,” she says.

“Teenagers are sexually curious, so it’s no surprise that crushes develop.”

Susan Quilliam says that same-sex crushes are also common - particularly for teenage girls. “They are vulnerable to crushes on their female teachers, as they’re often separating from their mum and are looking for new role models. They can find they admire their female teachers and want to adopt their character traits,” she says.

“For many girls it’s a phase, a way of exploring their fantasies. They may not be consciously attracted to the opposite sex then or in the future. But it can be unsettling for female teachers.”

Clare Harris knows. She considered resigning when a student crush got out of hand and her life started to resemble the film Single White Female.

“Sarah was a difficult student,” she says. “She had problems at home and needed someone to talk to. As her form tutor, I was happy to help and sat in at break listening to her problems. She seemed grateful, and her behaviour started to improve in lessons.

“Then she started appearing everywhere: my classroom, office, even took to waiting at my car in the evenings. She’d make up problems at home. I tried to steer her towards her head of year, but she didn’t want to know.

“Sarah copied my clothes, my hair, my gestures. My colleagues thought it was funny, but it stressed me out.

“The final straw came when she started hanging round outside my house. I’d arrive home from work and she’d be there, ‘just passing’. I threatened to leave before the scholl did anything.”

To curtail crushes, Susan Quilliam says teachers need to recognise the signs and take evasive action early.

“If you suspect a student has a crush on you, it has to be dealt with firmly. This is important, as many teenagers don’t realise that their behaviour is inappropriate. You have to give a clear message that this is going nowhere.”

This means treating the student in exactly the same way as others, making sure that you are never alone with them and that senior staff are aware of the situation.

Susan Quilliam also advises teachers to share concerns about crushes with colleagues. “Joke about it in the staffroom if you like, make it public,” she says. “That way, your colleagues will know you’ve nothing to hide.

“Students with crushes can get so desperate for attention, they end up making allegations about the teacher they fancy. If this happened and you’ve been open with colleagues, they will believe you and will support you.”

Although rare, allegations of improper relationships can have serious repercussions. “For teachers, pupil crushes are an occupational hazard,” says Chris Keates, acting general secretary of the NASUWT. “We’ve dealt with cases where embarrassed or hurt students who have had a crush on a teacher have made false, exaggerated and malicious allegations.

“These have resulted in teachers falling victim not only to child protection procedures but also to the provisions of the Sexual Offences Amendment Act which makes it a criminal offence for a teacher to have a relationship with a pupil in their school. Teachers should not treat them lightly, nor encourage them.”

* Names and some details have been changed