As the pupils stream in from the break, their buzz and chatter fills the air. The room barely seems large enough for the tide of red jumpers, but within seconds they have taken their place on the carpet. The chain of crossed legs stretches to every corner and apparent chaos dissolves into order.
After letting the excitement continue for a few seconds, Sarah Dorling picks up a pair of small cymbals attached by a string and strikes one against the other. The sharp "ting" brings swift silence and 71 pairs of eyes lock onto their teacher.
That's right, 71 pairs of eyes. For once, the impression of a vast sea of pupils is not far off the mark. These seven and eight-year-olds are believed to be in the largest class in the country.
But Ms Dorling seems unfazed. She embarks on the lesson as though there were only 30 children in front of her, not more than twice that number. The pupils sit quietly - even if the room is crammed almost to bursting, everyone has their own bit of carpet and there is no more noise than in a regular classroom.
This afternoon Year 3 are learning about family relationships. After Alex Jellings, the other Year 3 teacher, takes the register, Ms Dorling asks the children to help compile a list of family members on the whiteboard. If anyone is made shy by the size of the class there is little sign of it, as suggestions come thick and fast.
When the list starts to peter out, shortly after "brother's girlfriend", the teacher shows a photograph of her own family, before asking the pupils to split into groups. The floor clears and the children disperse into two adjoining rooms to discuss the family photos they have brought from home.
"I was quite apprehensive at first," admits Ms Dorling, who has taught at Bure Valley School in Norfolk for the past eight years. "I had never done anything like this before, but after a few days it seemed perfectly natural."
The supersize classes were dreamed up by headteacher John Starling. He arrived at the school, in the market town of Aylsham, near Norwich, four years ago, and quickly realised that space was a major problem.
The school's main building contained small classrooms separated from a large but under-used communal space by concertina doors that were kept almost permanently closed. Short of a major rebuild, opening up the doors and using the communal space seemed the best option.
"It was originally a response to the space: it was restricting the way we could teach," Mr Starling says. "We wanted to open it up and make it work more efficiently, and at the same time make the school more welcoming."
But it was not just about a better use of space. Mr Starling also believed that getting the classes to work together more closely would pay dividends, as both pupils and teachers learnt from each other.
The result is that what was once little more than a wide corridor is now a teaching space, with classrooms attached for when the pupils split into groups.
Supersize classes are nothing new. "Team-teaching" was fashionable in the 1960s, and many schools have large classes for individual projects. What makes Bure Valley unusual is both the numbers involved and that it is so entrenched in the school day. Although the children work in small groups, as they would in any class, large chunks of every lesson are spent together. Pupil-adult ratios are no higher than in most primaries. As well as the two teachers, each class also has up to three teaching assistants.
The debate about class size has moved on from the days in the mid-1990s when it was a touchstone of education policy. Research for London University's Institute of Education and the Organisation for Economic Co- operation and Development, among others, has suggested there is no clear link between class size and achievement. But the belief that children do better in small groups is hard to shift.
With no more evidence that his idea would work than a hunch, Mr Starling had to persuade teachers, parents and governors to get on board. But the results so far seem to have proved him right. Half-way through the year, the pupils had already achieved targets they had not been expected to reach until the end of the year. Although Mr Starling does not claim the large classes are solely responsible for this, it gave him the confidence to continue the experiment.
One of the keys to making it work was having teachers who were able to co- operate and jettison proprietorial feelings about having their own class.
"Sarah and I worked very well together," says Helen Pope, the Year 3 teacher who shared the class with Ms Dorling before becoming acting deputy head last term. "We have very different teaching styles but we have both learnt from each other."
Ms Dorling agrees. "You need to have people who are willing to share ideas and share the space, because teachers can be quite territorial," she says.
The supersize classes mean teachers pool much more than just classroom duties and planning, Ms Dorling adds. It also makes teaching more sociable. "We share expertise, bounce ideas off each other, chip in and help each other out," she says.
She likens the experience to a less stressful method of professional development than formal lesson observations. Watching another teacher and then discussing the lessons afterwards makes it easier to absorb suggestions when it is an everyday occurrence. It also means there is someone to chivvy her along when she is not having such a good day.
"It makes you more willing to try new things. When you have somebody with you, you can be a bit more open-minded," she adds.
Any fears she had about teaching a class of 71 have been swiftly dispelled. She soon got to know all of the children and realised which ones needed to be coaxed out of their shells. Some of them were reluctant to put their hands up at first, she says, but most seem happy to join in this afternoon's session.
It has been a steeper learning curve for Ms Jellings, her fellow Year 3 teacher who joined the school as a regular supply teacher at Christmas. Indeed, it was quite a task to find a supply teacher to take on the role, Mr Starling says. But Ms Jellings says that teaching a class of 71 feels little different from teaching 25. "There is a bit more noise when they're coming onto the carpet, but once they have settled they are pretty good," she says.
They differentiate in much the same way as in a smaller class, and the presence of up to five adults in the room means it is possible to check that every child is paying attention, she adds.
On the other side of a - soundproof - door, Year 4 are having a maths lesson with their two teachers. After Abby Lowe conducts the children through a bingo starter, Nicola Beevor takes over at the front of the class for the bulk of the lesson, on subtraction. This is also a supersize class, albeit slightly smaller, with 60 children. As well as the two teachers, three teaching assistants are placed at strategic points around the room.
This time all 60 pupils are together for about 45 minutes - half the lesson - before splitting into groups. Even then, the whole class reunites for short sessions twice more before the end of the lesson.
Although there is no division of specialism, the arrangement does allow teachers to play to their strengths. For Ms Lowe this is literacy and music, while for Ms Beevor it is science and maths. "We complement each other very well," says Ms Beevor.
As an NQT, she arrived at Bure Valley already used to sharing her class and working closely with another teacher on planning, teaching and assessment. "It is like having a permanent mentor," she adds. And far from being thrown in at the deep end, she says it was less daunting than having her own class of 30 with only a TA in the morning.
It is important to be willing to compromise, adds her Year 4 partner. "When you are shut in your own classroom you can do what you want," says Ms Lowe. "It is no good saying, `I do it this way, so that is how it should be done'."
She believes children make more progress as a result of having more pairs of eyes watching them, as well as having more minds thinking about what they need. And watching teachers collaborate sets a good example. "They see us working together and learning from each other," she says. "They're not afraid to ask questions and that is partly because they see the way adults interact."
This echoes one of Mr Starling's most cherished themes: that everyone in a school should be a learner, teachers as well as pupils.
Katy Cooper says her son Nathanial has thrived since arriving at Bure Valley in Year 3. "With a larger group of children, there are more of the same ability as him to work with, and he is not split up from any of his friends," she says. "They also mix them around so he works with different people."
As both teachers get to know all of the pupils, if a parent comes in and one is not available, the other can step in. It also allows teachers to talk to parents without bringing the lesson to a halt. "The fact that two teachers know every child is a huge asset," Mr Starling says. Consistency is another advantage, as all children are getting the same thing.
There are downsides. Nine-year-old Jessica says it can sometimes be difficult to hear what is going on, and even though every pupil has an allocated spot on the carpet there is still not much room. But it does mean that if she can't talk to one teacher about a problem, she can talk to the other. Abdi, also nine, agrees. "There are more people to help you," he says.
So far, lack of suitable rooms means only Years 3 and 4 have combined classes, although alterations now underway should allow it to extend to Year 5 in January, with Year 6 following soon after.
The experiment is now concluding its second year. It is still early days, of course, and is still evolving and changing. Mr Starling sees no reason why it should not be effective elsewhere - "What is good practice for 30 is good practice for 70," he says - but equally he makes no claims that it will work for all schools.
For now, it is enough that it works at Bure Valley, even if it was originally a way of using the space better. So far, it seems to have been a happy accident.