A solid-looking plastic crate brimming with books sits on the teacher’s desk at Shenfield High School. The girls - for they are all girls in this class - come up in groups to rummage through titles such as The Little Gymnast and The Little Yearnings of Annabel as well as more established works: The Hobbit, T S Eliot’s Old Possum’s Book of Practical Cats. In the stock cupboard is a second box where names like Trooper Jack crop up on the covers, along with the familiar goggles of that intrepid aviator Biggles.

The two boxes are not painted pink and blue, but the contents are clearly chosen for girls and boys. That the reading is aimed so specifically at the different sexes is possible because these Year 7 pupils are the first in the school - in Brentwood, Essex - to be split for every class into girls and boys.

According to Sophia Nelson and Lauren Kelly, both 11, the quiet and orderly atmosphere of their English class is almost entirely due to the absence of young men. “It gives you more confidence. They distract you because girls are ‘into’ boys. They hog the limelight. If they were here now, they wouldn’t be reading. They would be getting up and all meeting at the back to pretend to choose a book. Basically, they are naughtier.”

Their teacher Stan Baker prefers it this way too, and has a simple measurement to prove it. “In the time they will have filled two-and-a-half exercise books, they would only have filled one-and-a-half in a mixed group.”

It has long been held that girls do better in a single-sex environment. Education Secretary Gillian Shephard said as much last year, adding that boys do better in mixed classes because girls are a civilising influence.

Meanwhile, girls’ schools have been closing and co-education has been spreading - even in the private sector where an increasing number of leading public schools see it as a way to win pupils - clever, female ones. (Mill Hill School and Cheltenham College both announced co-ed plans earlier this month. )

At the other end of the performance spectrum is the crisis of under-achievement among young males. With boys lagging behind in higher GCSE scores by 9 per cent for the past two years, and in English by as much as 16 per cent in 1993, the spectre is rising of a generation of young males entering the adult world barely literate and unqualified for basic jobs.

At Shenfield which serves a reasonably affluent area, the overall results are well above the national average. Yet teachers here are sure boys also benefit from single-sex teaching because, as one of them puts it, there is no “peacock effect”.

Stan Baker admits it is hard to inspire a group of bored, low-ability boys in English, but describes his own technique with sardonic aplomb. “You win them over by appealing to what interests they have, such as football and martial arts.” They discuss the lack of local facilities for skate-boarding and mountain bike riding. “You can use the peculiar patois which boys have. ”

According to Olive Baldwin, Shenfield’s head of English, targeting reading is crucial - “if there is a picture of a girl on the front they won’t read it” - although her department does ensure that a wider range of works is used in class to avoid a Biggles-only diet and questions about gender in literature are tackled.

Shenfield’s decision to introduce single-sex teaching, starting with last September’s intake, caused uproar when it was announced in November 1993 with coverage in many national newspapers. “We were puzzled by the level of interest at first,” says Dr Peter Osborne, the headteacher, “but it’s because single-sex schools have been closing for 25 years to make mixed schools and now we’re going another way.”

The school’s interest in gender stretches back to 1985 when it launched a project to involve more girls in science. Instead of allowing them to sit at the back of physics classes and watch the boys, they were sat at the front and encouraged to answer questions; laboratories showed posters depicting women scientists. Dr Osborne recalls that up to a third of girls opted for physics as a GCSE, although many later changed their minds because of parental pressure. “Their fathers told them it was not a girl’s subject,” he sighs.

Nevertheless for the duration of the experiment, while the girls flourished the boys did not. “They behaved differently but in a challenging way.”

The experiment was deemed to be unfair to boys and Dr Osborne became more convinced that a more permanent segregation of the sexes for all subjects was the answer. For evidence, and by far the biggest influence on Shenfield’s subsequent decision, Dr Osborne looked up the road - 10 miles up the A12 to be precise - to Moulsham High School in Chelmsford. Here the sexes have been taught separately - except in non-core subjects at key stage 4 and in the sixth form - for 20 years, reflecting a desire by parents to maintain in some form the boys’ and girls’ schools which formed it.

Moulsham High should be the statistician’s wonderland for assessing single-sex classes but the picture is not straightforward and researchers have not rushed to the scene. (More surprising, perhaps, is researchers’ apparent lack of interest in what is happening at Shenfield this year. Beyond a parent’s interest for a master’s degree, no outsiders are studying the effects. “We’ve had no attention from OFSTED either!” says Dr Osborne.) At Moulsham from 1988 to 1992, the differences in GCSE results at higher grades between boys and girls was lower than the national average and appeared to be falling (girls were 0.7 per cent ahead in 1991 and 1.1 per cent better in 1992).

But the past two years have seen a sudden jump to equal the national gap, about 10 per cent, although the combined total of 52 per cent higher passes is itself a good result. A divide of between 5 and 7 per cent is expected this year between the girls’ and boys’ results.

“Dividing them per se may not have the effect you hope,” comments Moulsham head Dr Chris Nicholls. “It is what you do when you have them divided that counts.”

With so many variables, easy answers - indeed any answers - are difficult to find. Single-sex teaching means smaller classes, especially when children are streamed academically as they are at both schools.

How much do these factors influence the overall results and do they effect one sex more than the other? Would success in aspiringly middle-class Essex be repeated in a poor urban area or a very affluent farming community? All such questions await the number-crunching of academe on a bigger scale.

At Shenfield the real test will come in four years when the first cohort to be taught separately will sit their GCSEs. But already the headteacher is brimming with anecdotal success.

“It’s going better than ever I could have expected, ” says Dr Osborne, who retires at Easter to be replaced by Moulsham’s deputy head, John Fairhurst.

His staff, he says, are a pragmatic bunch “not ones for jumping on a bandwagon”. The change to single-sex teaching was an act of faith for many of them but, after only two terms, “I’m getting an enthusiastic response from people I would never have expected it from”.

There are great advantages, he says, in pupil behaviour and attitude to work stemming partly, he thinks, from separating the environments of work and play. “The pupils seem to understand that lessons are for working in, while at break they can socialise and mingle freely...

“I’m not advocating single-sex schools, or single-sex teaching in primary schools and sixth forms. But for our pupils the opposite sex is suddenly becoming overwhelmingly interesting. That’s fine: the opposite sex is there at break time and at all school societies. We are just keeping them apart for the silly years.”

The governors’ decision to segregate co-incided with a successful application to go grant maintained. Dr Osborne saw the opportunity to create a new “lower school” out of Years 7 and 8 which he felt were an age group needing more direction - “Year 9 are thinking about GCSEs and in Year l0 they’ve started them”.

This restructuring was intended to create new social opportunities for the two year groups and already a lower school council and charity appeal have gone ahead. This new identity - and the harnessing of Year 7‘s natural enthusiasm - is held up as an important part of the plan to raise achievement.

At Moulsham, Dr Nicholls makes the same distinction about children’s need to socialise outside the classroom. “Schools are not natural places. Learning is natural, but schools are artificial situations. We should bury our ideological standpoints. Boys are finding it more difficult to find their way, and their role in society.”

In talking to pupils at both schools, a few home truths emerge. The first is that they all seem to like it, which cannot simply be explained by pupils’ natural loyalty to the school of their choice. The second, accepted by both sexes, is that it is the boys who cause classroom disruption.

The only pupils who disagree are those who, in their own words, thought that “messing about in class is a laugh”. A surprising number of boys - apart from saying they behaved better when separated - found it easier to express themselves in their own company. “You can just let yourself go and say what you think,” said one Shenfield 11-year-old recalling all-male drama classes. “You can go for it without thinking anyone will laugh at you.”

This is evident in a Shenfield French lesson, where boys are responding to the jolly male stimulus of competition, in the form of a picture noughts and crosses which can only be filled in when a French sentence is correctly spoken. Hands fly up in eagerness to have a go and although the accents are more Essex than Elysee, the answers are readily given. Mistakes, although corrected, are soon forgotten. There is little bashfulness.

Shenfield staff talk quite freely about different strategies for teaching male and female classes. Boys they say, respond to short-term goals and visual rather than written stimuli; teachers can be far sterner and use sarcasm to more effect with boys than with girls. Girls, on the other hand, can play a longer game, take in more written material and can be admonished without voices being raised.

“There is a simplicity about all this which is very appealing,” says Dr Osborne. “It is much easier to address boys’ problems in boys’classes - we can develop strategies and apply them.”

In a world where money follows pupils, one factor cannot be overlooked: parents like it. In four years of open enrolment at Moulsham, the roll has risen from 1,230 to 1,500. At Shenfield, which opted out in April last year, applications have risen by 70 per cent since the new intake was split. Interest is also shown from other schools.

On the day The TES visited Moulsham, Dr Nicholls was also entertaining a head and deputy from two inner London schools who saw segregation as an answer to discipline problems as well as to improving exam results.

But not everyone is a convert. Dr Alan Leech, head of Bohunt Community School in Hampshire and a firm advocate of co-education, says such a system would be damaging at his own school.

“For a co-ed school to segregate runs counter to what it is there for. Why not take it further and separate them at break as well? They are learning there as well. Education isn’t just what happens in the classroom.”

He says that boys will respond if schools raise their expectations of them - by ensuring they speak in public, answer questions in class and take part in assemblies. Bohunt’s statistics, he says, prove the point, with 57 per cent of both boys and girls scoring five or more higher grade GCSEs last year, with a similar outcome in 1993.

Dr Madeleine Arnot, of Cambridge University’s education department, says more research is needed before the jury returns on single-sex teaching. One project in the 1980s that set girls aside for maths had initially encouraging results, but that was mostly to do with the extra attention they received; there was little long-term effect. “The fact that schools are recognising that there is a problem is more important than what they actually do about it,” she says.

In Denmark, some schools separate girls and boys for a few periods to allow them freer expression in a single-sex environment, but that is for personal rather than academic development.

John Fairhurst, Shenfield’s head-in-waiting, admits there is insufficient evidence of improved academic performance to cry Eureka! “but parents have a gut feeling that this is a sensible way to do things.

“It is a recognition that boys and girls are different and that they have different needs. The social case for co-education is powerful, but this is the best of both worlds.”