The girls are awkward and self-conscious. The taller, larger ones hunch their shoulders and shuffle along the school corridor, as though embarrassed by their own physical presence.
The boys, meanwhile, are particularly interested in just these girls. They snigger behind their backs; they fantasise about the girls' newly developed bodies. Not too much, though, and not in public: their own bodies are far too treacherous for that.
All of them are moody and irritable. They dislike their friends, their parents and their teachers. Most of all, they dislike themselves. Welcome to the primary-school classroom.
Increasingly, problems that were once the preserve of Year 9 teachers are infiltrating primary schools. Teachers who went into the job assuming they would be working with wide-eyed children who still put adults on a pedestal now find themselves faced with sulky, moody proto-teens.
Today, the average age of breast development - the first sign of puberty in girls - is between 10 and a half and 10 and three-quarters. This is a significant advance on the 1950s, when a study put the average age of puberty at 11 and a quarter years.
And menarche - the age at which a girl has her first period - offers further evidence for the advancement of puberty. Since 1900, this has dropped steadily. "The classic definition of early-onset puberty was anything before age eight," says Peter Hindmarsh, professor of paediatric and adolescent endocrinology at London's University College Hospital. "In the 1950s, that was unusual. Although we still operate on that definition, we probably should drop it a little. Puberty is becoming more common at age eight."
But puberty is about more than merely physical change. Previously compliant children transform, apparently overnight, into cauldrons of simmering hormones.
"I think there's a genuine behavioural issue," Professor Hindmarsh says. "In boys, testosterone levels developing in the brain is really bad news. They start having feelings around girls. They will have erections around girls. They often become quite inappropriate in their behaviour around girls in the classroom."
In other words, they start behaving like teenagers. "We're dealing with mood swings more and more," says Ian Bradbury, head of Danesfield Church of England School. "We'll say, `they're getting hormonal', `they're getting awkward at home.' But we're talking about nine- and 10-year-old children."
Sharon Deackes agrees. "You might get them arguing with their friends at school and you think, `They never used to do that,'" the head of St Mary's Primary, in Leicestershire, says resignedly. "Or they start saying, `I don't want to do that.' They're having strops; they don't want to do things. They're just awkward."
But there is much more to teenagedom than mood swings and a few well-timed I-hate-yous. If primary pupils - like teenagers - feel under constant physical and emotional onslaught from their own bodies, it is schools' responsibility to help ease the pain. Dealing with miniature teenagers means avoiding being caught unawares.
"You do hear the horror stories," says Chris Davis, of the National Primary Headteachers' Association. "Girls who hadn't been prepared in any way, and were horrified when they found blood in their beds or in their knickers. They thought they were in terrible, terrible trouble. I've heard stories of suicides. They're apocryphal, maybe, but it's something that enlightened staff do need to try and avoid.
"That's why it's very important that the subject is broached before all the children - or most of the children - encounter things for themselves. It's definitely happening earlier than it was."
"Years ago, when I first started teaching, you wouldn't even anticipate a Year 6 girl starting her period," agrees Ms Deackes. "Now, there are Year 5 girls who look very, very mature, physically. You think, `It's not going to be long before it's you, too.' You need to look at having sanitary- disposal bins in the bathrooms - that's not something we ever had before."
And then there are all the other comfortable realities of primary-school life that suddenly need to be called into question. "Previously, for years and years and years, older children all changed for PE together," says Ms Deackes. "Now, we have to have them changing separately: boys in one classroom and girls in another.
"And supervision has become a problem. If you have two male teachers in that year, then you have to bring in another teacher to supervise the girls. You can't leave them unsupervised. But to put a man in there defeats the object."
Mr Bradbury, meanwhile, has had to introduce individual cubicles into the girls' showers. "The sheer height difference and shape difference between your taller, bigger girls and your very short, little girls is quite striking," he says. For example, there is a one-foot difference between the tallest and the smallest girls in Danesfield's Year 8. One looks as though she is 15; the other resembles a nine-year-old.
"There's a lot of self-consciousness coming in, lowered self-esteem, people who've conveniently forgotten their PE kit," Mr Bradbury says. "Where that used to be 14 to 15-year-old girls, now it's getting earlier."
While there may be greater potential for a rising swell of embarrassment among early-developing boys, male early-onset puberty is comparatively rare. For girls, the consequences are no less disturbing for being more subtle. "When you're superimposing a lot of female hormone on top of a brain that's not ready to process it, it causes a lot of confusion," Professor Hindmarsh says.
"They see themselves as essentially age-appropriate, but there are a lot of changes taking place. It doesn't sexualise them in an overt way - they're not parading around like something off the Milan catwalk. Instead, they become slightly more withdrawn individuals, who are more self- conscious about themselves."
For Chris Davis, this is a familiar sight. "It's the ones who develop early who are self-conscious," he says. "Their posture changes. I can picture to this day girls who covered their shoulders or stooped - covered up the fact that they were developing."
Indeed, how one looks is increasingly important. The public demonisation of obesity has led to a growing sense among primary schoolgirls that becoming fat would somehow signify moral, social and intellectual decrepitude. But the pre-adolescent mind can lack the sophistication to distinguish between excess weight and pubertal ungainliness.
"A lot of girls grow up feeling fat, especially if they develop earlier," says Diane Christopher, the recently retired head of Sandown Primary, on the Isle of Wight. "The media makes them feel that they need to be and remain flat-chested and thin."
In a world where supermodels have the figures of prepubescent girls, being a prepubescent girl with the figure of a supermodel can make life particularly difficult. Name-calling, for example, is a huge problem.
"If puberty hits earlier, it adds another thing for girls to be picked on and teased about," says Tom Narducci, senior consultant for the NSPCC. This can happen at home, as well as at school: superficially benign comments such as "you're getting big now" can be misinterpreted by pubertal proto-teens.
"Those sorts of comments are going to affect an eight- or nine-year-old's view of things," he says. "If burgeoning sexuality is made a reason for mockery, then it has the potential to be quite harmful to self-development and self-esteem."
Oversensitivity is hardly a rare quality among hormone-addled pubescents. But these are not door-slamming 14-year-olds. They are door-slamming nine- year-olds in the bodies of 14-year-olds.
"You have an eight- or nine-year-old, with the cognitive ability of a child," says Mr Narducci. "That's a very difficult place for a child to be: they're getting mixed messages from society, but they're also getting mixed messages from themselves.
"Those hormones are an additional level of pressure, coming from within. There's enormous pressure for girls to look good, to look attractive, to look sexual. It's much, much harder to say `that's not for me yet' when the message you're getting from your own body is, `yes you are ready'."
Academics at London's Institute of Education have shown that young girls believe that pleasing boys is more important than ensuring their own happiness or pleasure. Girls barely into their teens use words such as "slut" or "whore" to describe themselves on social-networking websites. Primary-school girls are already dressing in crop-tops, leggings and Playboy-branded T-shirts. Add breasts, hips, and a lack of emotional maturity and you have a situation ripe for unscrupulous exploitation.
"I've seen Year 6 girls when they're dressed up for the school disco," says Ms Deackes of her pupils at St Mary's. "And I think, `Oh my goodness, you look about 14.' I worry that someone will take advantage of them."
And primary-aged pupils often lack the sophistication to realise the potential implications of premature sexual activity. "Then what you're left with is victims," Mr Narducci says. "You have very young children vulnerable to people who might exploit them for their own ends."
Ms Deackes refers with a shudder to Elena Chiritescu, the Romanian 10- year-old who gave birth to a daughter this November. "When you read about that, you do think, `Oh, no,'" she says. "We do talk about personal safety - that people shouldn't ever put expectations on you, and that it's OK to say no. We always talk about sexual relationships happening in a loving situation, and we stress that it shouldn't happen below the age of 16."
Here, too, primary teachers are being forced to confront issues that were previously the preserve of secondary schools. At Sandown Primary, for example, Mrs Christopher decided that it was time for a review of how, and when, sex education was delivered.
As staff meetings go, this was one of the more unusual ones. The teachers sat down in front of the TV, and Mrs Christopher switched on the video; a naked cartoon child walked onto the screen. As the child stood there, the programme began labelling different parts of her body, offering a simple explanation for each part. First there were the nipples: these, the programme informed viewing staff, would change shape in puberty. Then there was the vagina. And then there was the clitoris.
There was a sharp intake of breath, followed by a couple of giggles. "Whoa," someone said. "For eight-year-olds?"
But Mrs Christopher had no doubts at all. "We had to introduce sex education in a more explicit way," she says. "More and more children are becoming sexualised at an earlier age. There were embarrassed sniggers among the adults, but no one said, `Why are we talking about that?' We're peeling back some of the mystique."
But no one goes into primary teaching hoping to work with teenagers: for that, there are secondary schools, with their classes full of actual teenagers. Suggesting that primary teachers deliver the types of lessons once considered the preserve of secondaries runs the risk, therefore, of inciting staffroom rebellion.
"I would never ask my staff to teach something I wouldn't be prepared to teach myself," says Ms Deackes. "They know that. And, for any staff who may not feel quite happy about teaching certain subjects, I'd say, `How would you feel if you hadn't done this lesson, and then you have a child who starts her period in your class?'"
At Danesfield, Mr Bradbury takes a different approach. The school uses mixed-age tutor groups, with pupils between Years 5 and 8 all taught together. "That way, the child who's more developed doesn't always stand out," he says. "They're with older children, anyway."
Growing up becomes less daunting when pupils are already sharing lessons with older, more grown-up classmates. But there is also less pressure to become prematurely sexualised when nine-year-olds see 13-year-olds still playing the same games that they enjoy themselves. "They think, `I can still be me,'" Mr Bradbury says. "They don't have to do something they don't want to."
Play is, in fact, an important part of his strategy. Instead of treating his primary pupils like underaged teenagers, his teachers treat teenagers like oversized primary pupils. "We very much have a culture of children are children," he says. "Our Year 8s still make daisy chains in the summer. Games and old-fashioned play are very much a part of the ethos. We've worked so that children don't grow up too quickly."
But, he acknowledges, this approach works better in his isolated Somerset school than it might in an inner-city primary. Indeed, the best way to help premature pubescents is to remember the key quality of all teenagers, whatever their age: profound selfishness. Their own needs dominate their lives; it is these very needs that teachers must try to address.
"We probably need to do more, in terms of recognising what our children's experiences are," agrees Mr Narducci. "I don't think there's a one-size- fits-all answer.
"Put yourself in your children's shoes, growing up. What support and teaching would be useful? What message do they need to help make sense of their internal and external worlds? We need to make this less scary, less mysterious. We need to make their experience something positive and constructive, rather than negative and damaging. It's all part of growing up."