"Just to warn you," says Marina Coleman, headteacher of St Vincent's RC Primary School in Marylebone, London. "The children will be sitting in rows."
Two Chinese maths teachers have flown 5,500 miles to demonstrate calculation, so the 29 children in Year 2 have had their desks arranged Shanghai-style for the occasion.
Li Dong, 29, from Luwan No 2 Central Primary School in China's largest city, stands in front of the six-year-olds. He is dressed in a suit and tie and beaming at everyone. He gestures to the class to rise.
"Good morning, class," he says. The Year 2 pupils, in their dark blue jumpers, stand and chant: "Good morning, Mr Li."
Mr Li is in London as part of the Shanghai Teacher Exchange Programme, a key element of the government's pound;11 million maths hub scheme, in which 30 English schools have been set up as centres of excellence.
Teachers from schools within each hub travelled to Shanghai in September to observe lessons, and now Chinese teachers are making the return trip.
In his lesson today, Mr Li will look at addition. He jumps straight in with a three-picture cartoon strip, showing a picture of three children in a car, another child getting into the car and the car with a big question mark.
Arms shoot into the air - three plus one equals four. These children have had two years of schooling already; in Shanghai at this age, they'd only be weeks in.
But despite the disparity in formal schooling, the level of comprehension is about the same in both countries. The calculations in Mr Li's Shanghai maths textbook are kept simple so that children focus on the aim of the lesson - recognising which problems require addition.
Half an hour earlier, he was going through the lesson plan in St Vincent's basement meeting room with 11 visiting teachers from schools around Westminster.
Mr Li explained to the teachers that in his previous lesson at St Vincent's, he covered using addition to combine two groups of objects. Today's lesson would be looking at using addition when you start with one group of objects, adding another group later. It is a nuance, but one that helps to deepen children's understanding of the concept. The calculations, lots of them, will come later.
Mr Li finished his explanation, packed up his small white flask and headed upstairs to demonstrate.
Afterwards, he talked about the lesson. "It is not that the children are different in Shanghai," he said. "You also have children who know the answer. But in Shanghai a lot of time is spent asking children to explain their answers. They've got the answers, but what I want to know is how they got them.
"Now, in Year 2, it seems very easy. But in Year 5, a question may have a lot of different methods and it is important to know how to get to an answer."
Katrina Hassan, a Year 2 teacher and maths coordinator at St Vincent's who visited Shanghai last month, said she also noticed how the numbers zero and 10 were brought into every lesson to reinforce their importance. In this case, having no children on the bus had stumped one child - something that was noted for future work.
"In Shanghai, it's taught in such a carefully phased way," Ms Hassan said. "Here, we accelerate them quickly but there are gaps in key stage 2."
The teachers attending the masterclass were impressed with the depth of learning that Mr Li and his colleague Mr Chen expect from pupils - each lesson is pacy but learning is consolidated over the weeks, and teachers are allowed to keep children behind once a week for extra maths if they have not understood a concept.
But Mr Chen pointed out that his job in Shanghai was not the same as a primary school teacher's in the UK.
"The biggest difference is that in China every teacher only has one subject so they have much more time to mark and prepare lessons," he said. "In England every teacher is very busy. They have many subjects so they have little time to do other things. This is very different from China."
He added: "In England, the lessons have many activities, which are very useful. Maybe this is something we can use in China from England."
Stephanie Keenan-Logue, a maths coordinator at St Edwards RC Primary in London's Lisson Grove, said that going back over ideas in different ways was a very useful teaching strategy, but pointed out that the curriculum in England had a greater focus on covering content. Nevertheless, she added, "it's an amazing experience. I've learned a lot to feed back to my school. It's one of the best courses I have been on."
Fellow London primary teacher Dolly Tobin said: "For a school to experiment with a method like this - where we say `We're going to go very deep and maybe we'll get better results in the long run' - would be taking a chance in the current climate. Schools have the pressure of Sats at the end of Year 2 and Ofsted.
"We don't have time to go into that depth, even though in the long run it means in England we do have a bunch of children in Year 5 who don't know their times tables."
And what did the children think? Alessandro Castagna, 7, was impressed with Mr Li's lesson. "I liked the bit where the children got up and pretended to be on a bus," he said.
Ella Wise, 6, liked spotting the patterns within the sums at the end, and didn't mind the desks being moved from groups into pairs. She added: "I liked standing up for the teacher because it's polite and kind."
Meanwhile, Mr Li said he was enjoying his time in England. He had been warned about the food, he said, but had discovered that he quite liked sandwiches. What he had not been prepared for was drinking cold water.
"Always the water is cold," he said. "If you want a hot drink you have coffee or tea, not water. So that is what is in my flask - hot water."
Shanghai exchange scheme: your questions answered
What is it?
An exchange programme for maths teachers from England and Shanghai.
In September, 71 teachers from the UK spent two weeks observing lessons in China. Now their Shanghai colleagues are making the return trip. Currently, 29 are in the middle of a three-week stint in selected UK primary schools. Another 34 are due to visit next term.
Why is it happening?
Shanghai topped the 2012 Programme for International Student Assessment (Pisa) league tables in maths, whereas England came 26th. Shanghai 15-year-olds are about three years ahead of their peers in England.
How is maths taught in China?
Shanghai teachers emphasise establishing core skills, such as multiplication tables, at a young age. Children spend their first three years at school embedding this knowledge.
Subject specialists take the classes, and every pupil is given the same lesson rather than being split into ability groups. The students are expected not just to provide answers but to explain their reasoning.
Teachers in Shanghai spend less time teaching and more time reviewing and planning lessons than in England.