It was breaktime and Stephen Brierley was hoping to catch up on some marking. He was a new teacher in an 11-16 comprehensive and keen to make use of every spare moment of preparation time. He did not, however, make it as far as the classroom.

A prefect stretched out his arm. “You’re not allowed in here,” he said.

“I’m sorry?” Mr Brierley replied, baffled.

“You’re not allowed inside at breaktime,” the prefect said.

Then the penny dropped. “Oh. No. No, no,” Mr Brierley said. “I’m a teacher here.”

The prefect smiled wryly. “Yeah, yeah,” he said. “That’s what they all say.”

Youthful looks are generally seen as an unequivocal blessing. The occasional moment of awkwardness at the bar or off-licence is easily outweighed by the pleasures of provoking jaw-dropping incredulity (“Really? No! No way that’s your real age.”) Not to mention the money saved on Botox.

But for teachers, being baby-faced is not always an advantage. Being mistaken for an 18-year-old is all very well when your workplace includes people of all ages. It is something else altogether when you work in a school full of actual 18-year-olds.

Hazel O’Neill is all too aware of the potential for embarrassment. “I was told off by my headteacher,” she says. The Hertfordshire RE teacher had just left the school and was walking down the road, sending a text message. The head called her to task: mobile phones were not allowed in school.

“What on earth do you think you’re doing?” he said. “You know you’re not allowed phones in school. I’ve a good mind to confiscate that.” His manner did not make Ms O’Neill‘s explanation any easier. But once she pointed out that she was not a pupil but a teacher, the head looked at the ground, mumbled an apology and speedily made his excuses.

She is not the only young-looking teacher who has been called upon to explain herself. It will be a rare member of the kitchen staff who has not upbraided a teacher for pushing to the front of the queue, a rare caretaker who has not sent a sixth-former away from the school car park only to discover that she was actually a member of staff.

“When I go into the resource room to get things photocopied, people always say, `Who’s that for?’” says Emma Hyner, a trainee RE teacher in Waltham Forest, east London. “And I say, `Erm, for me.’ But they’re always very apologetic. It’s just one of those things.”

Sara Bubb, senior lecturer at London University’s Institute of Education, points out that young teachers should not be too worried about the occasional telling-off from another member of staff. “I’m quite envious,” she says.

More awkward, though, is being mistaken for a pupil by the pupils. “The first time I walked into my sixth-form class, they asked me if I was new,” Ms Hyner says. “I said `Yeah’ and they asked me where I was going to sit. It kind of got us off on an odd footing.”

It is a situation familiar to Ms O’Neill, too. On a school trip with pupils she did not teach, she was once forced to threaten a misbehaving group of teenagers with detention. “You can’t give us a detention,” they retaliated. “You’re a sixth-former.” “I had to whip out my staff badge before they believed me,” Ms O’Neill says. “Thank heavens for the staff lanyard, I say. The day that I forget it, I’m in trouble.”

But looking like a Year 8 in a suit - as David Miliband was famously dubbed by the president of a teachers’ union - need not be a career hurdle. As a young - and young-looking - teacher herself, Sara Bubb deliberately went out and bought several Miss Jean Brodie-style outfits for school. “I wanted to separate out Miss Bubb from Sara Bubb,” she says.

“Stepping into imaginary shoes is quite helpful. It’s putting on the persona of a teacher. Where you might banter with friends, you need to avoid bantering with pupils. If you have a youthful giggle that makes you look younger - try and keep that back. Have self-awareness of how you come across to people and then try to compensate.”

This is something Ms O’Neill has found herself doing instinctively. Without a staff badge to protect her, she says, she would instead walk down corridors with a pile of exercise books in her arms, concealing her youthfulness behind the props of a teacher.

“I’m only 22, so I have quite a bit in common with the people I teach,” she says. “I have to put on an act and not talk about anything that isn’t to do with what I teach.”

For example, while she will allow herself to discuss moral conundrums in Hollyoaks or other soaps, she tries to give the impression that she watches these purely in the interests of professional research. “If they’re just talking about some rubbish reality programme, I have to not engage with them at all,” she says. “Give them the impression that I just watch the news and that’s it.”

She also adopts other strategies to make herself seem older. “I’ve found myself saying things like, `Years ago, when I was at school’ or `You’re probably too young to remember’, just to throw them off track a bit, make them think I’m older than I am.”

The need to make this distinction is the serious side of looking young. “You’re not a friendly sixth-former, a friendly older brother,” says Mr Brierley. “Make sure that there’s a very, very clear dividing line.”

He eventually managed to overcome his career beginnings as an unconvincing adult and is now head of The Deanery High in Wigan. “You can’t even afford to be avuncular in this situation. What you do in the evening, whether you’re going out with someone - it’s all off-limits.”

The best piece of advice he was given, he says, was to dress smartly. He had been wearing a jumper and slacks. As soon as he started wearing a suit, he noticed a difference in pupils’ responses to him. “If someone dresses informally, perhaps they’re more likely to get an informal response,” he says.

“I always go out of my way to dress conservatively,” agrees Ms O’Neill. “At the weekend, I let my hair down and I don’t always wear my glasses. But at school, I always tie my hair back, I always wear my glasses, I’m always buttoned up.”

Of course, caution is advised here: there is the (possibly apocryphal) story of the young teacher who chose a deliberately grown-up outfit of black skirt and white blouse, only to find that she was dressed in school uniform. But having a separate, teacherly costume can help young teachers assume a matching teacher persona.

“Get a different set of clothes,” says Ms Bubb. “If you’ve been a student, you’ll have one set of clothes. Things to avoid are dressing young and showing too much flesh.”

The latter, in particular, can be a problem for young-looking female teachers. Being seen as a teenage girl by the boys in your class is not the path to smooth behaviour management.

Sophie Canfield, a science teacher at Wrotham School in Kent, is just over five feet tall and often mistaken for younger than she is. “They do ask how old I am and how long I’ve been teaching,” she says. “They think I’m a similar age to them. When I say I’m 25, they’re quite shocked. I’m actually 10 years older than some of them and they’re all, `Woah! I thought you were younger than that.’”

As a result, she has found herself attracting a level of interest from male pupils denied to older-looking teachers. Some - fresh from a teenage growth spurt - will point out that they are tall enough to rest their arm on her shoulder. Others are more obvious in their flirtation: “Miss, Jason thinks you’re fit.”

“I can turn it around in such a way they get embarrassed by it,” Ms Canfield says. “I say, `Excuse me, do you want to repeat that?’ and they get very embarrassed. And sometimes I just choose not to listen. If they’re doing it to wind you up, and it’s not going to get to you, then obviously they’re going to drop it.”

Indeed, says Ms Bubb, this is a point at which teachers should present an unsmiling face. “The temptation is to laugh or banter,” she says. “But just think: what would Miss Jean Brodie say? She’d probably just give them a slightly arched eyebrow and put them in their place.”

“Establishing control is a lot about body language,” says Mr Brierley. “How you act carries a lot of clout: standing upright, tone of voice, commanding the space. Then pupils will revise their opinions very quickly. You might look like a sixth-former, but by golly you mean business.”

Baby-faced teachers should not forget that there are advantages to a youthful appearance. It is extremely unlikely that teenage pupils see Ms O’Neill as a mother figure. As a result, when issues such as abortion, sex before marriage or life after death come up in her RE lessons, they are more willing to be frank with her.

“They’re quite forthcoming with their questions,” Ms O’Neill says. “One teacher commented, `Oh, they’d never have asked me that.’ With an older teacher, they might have shied away from it more.”

And, of course, the curse of youth is fleeting. Indeed, a few years in the profession could provide an antidote to youthful looks. But few young teachers will be looking wistfully in the mirror for that first wrinkle or sprouting of grey.

“Eventually I’ll get older,” says Ms O’Neill. “But once I’ve got more experience, I won’t think about the age thing. So I won’t wish away my youth.”