"Hamstrings," cries a voice amid the ows and ouches, ooghs and aaghs.
"That's right," he enthuses, uprighting himself. "Now do a little shake of the legs so the muscles get nice and loose again." Cue wiggling of bare feet and spindly legs.
It may seem like any other gym class in any other inner-city primary school. But this is different. The motley collection of crew-cropped lads and pony-tailed girls of Chevassut Primary School, Hulme, Manchester, are getting a treat. For Jason Beckford is no ordinary teacher. In fact, he is not a teacher at all.
He is a former professional footballer with local first division club Manchester City, and was once a graduate of the Football Association's highly select national school of excellence at Lillieshall. After a career-ending knee injury, Beckford is now one of the club's football in the community staff and is here with two colleagues putting the latest stage of its ground-breaking "learning through football" scheme into practice.
The programme started about three years ago when Alison Vaughan, at the time a volunteer with City's community team, devised a way of making the club's contact with schools more relevant to the children's all-round education. A qualified teacher, and now City's full-time education officer, Alison Vaughan realised that football could be used for cross-curricular teaching.
Whereas coaches from professional clubs have often gone to schools to teach football skills - often filling the gaps left when school-organised sports coaching began to decline - she saw that football could be used to motivate children in a host of subjects, from English, science and art to health and physical education.
"Lots of kids get disillusioned in school," she explains. "Many are obsessed by football. We aim to use that obsession to make them interested in learning."
Alan McCleary, headteacher of Chevassut school, agrees. "It is great for the kids," he says. "They get three adults here spoiling them for an afternoon, they get me off their backs for a bit. And to cap it all they're footballers, which adds to the excitement."
With the help of Des Coffey, a school inspector and City fan, Alison Vaughan wrote a workbook for primary children and invited teachers to bring pupils to the ground for an education day. They learn about the stadium and the pitch, and the way the club works, while they study. "We can cover every angle of the curriculum, even religious beliefs," says Des Coffey.
Sitting with 25 11-year-olds beneath the main stand at Maine Road, City's stadium, it's clear that biology, anatomy and physiology suddenly have a previously unrealised interest when they're about a footballer's training, his diet, what injuries he might get, and how these are avoided.
Alison Vaughan describes the pre-match meals of children's favourite players, bringing astonished "urrghs" from the pupils of St Chrysostom's in Chorlton-on-Medlock. Terry Phelan, an old favourite of City fans, eats Weetabix and bananas before a game, she explains, while England striker Alan Shearer is lost without his chicken and beans.
"Why do they eat those foods?" she prompts. "Why don't they eat fish and chips or steak?" "Protein, " suggests one young enthusiast. "No, energy," insists another. Within minutes she has them listing other sources of "the footballer's fuel" - fruit, vegetables, pasta, rice. All the lessons are reinforced by workbook exercises. In one, pupils are asked to write a lunch menu for the players to eat before an important match and list "five things you would not recommend for healthy living".
Later Alison Vaughan stands them beside the team dugout, looking at the expanse of green that the club's physiotherapist sprints across half-a-dozen times every Saturday afternoon, tending to the latest fallen star. The workbook lists 10 common injuries, from hamstring and groin strain, to broken nose and in-growing toenail.
Children must label the correct parts of a picture of the body with the number of the appropriate injuries. They are asked to search newspapers for the names of injured league players to identify that injury and how long it will take to heal, as well as to describe "a programme of exercises that a player could follow to make sure they have warmed up every part of their body".
They also get to design a new strip for the team, to work out league tables from a set of results, and to compare match reports of a first team game that they all get free tickets to attend.
"It is genuine educational work which is great for helping them to learn basic skills," says Alan McCleary of the curriculum material, "and it helps me because the connection with football gets them interested."
According to Alison Vaughan, the girls respond with most enthusiasm. "They tend to be less cocky about what they know already," she says. While other top clubs have similar programmes - Aston Villa, for example, has an educational project, and Alison Vaughan has written a workbook for Blackburn Rovers, as well as helping Liverpool and Everton get education work off the ground - City's is the first to gear itself to the needs of the national curriculum.
Vaughan hopes the latest project, a drugs awareness initiative called "Kick It", will visit every primary school in the city. The club already has contact with 20,000 children a year, and considering its rivalry with footballing giants, Manchester United, the education initiative has marketing and promotional advantages.
"City do much more of this work than United because if we didn't almost every kid in town would be wearing United shirts," admits Des Coffey.
Yet, there is a belief in football's role as an educational tool. Starting next month, the programme will expand to secondary level again carefully following curriculum guidelines, and even building in performance targets. A City and Guilds certificate for teaching 16 to 25-year-olds is under discussion.
The enthusiasm for learning they generate was obvious at Chevassut school,where a class discussion on drugs followed the games, touching on drugs in sport, and those encountered on the streets near where the children live. Jason Beckford and colleague Barbara Williams encourage pupils to reveal their knowledge. Even in Alan McCleary's watchful presence, they talk of their contact with drugs, using slang and street names in a surprising and frank way. Barbara Williams carefully contrasts the perpetual highs and lows of drug addiction with the natural good feeling the kids are still experiencing from the exercise earlier.
"It was a brilliant session," says Alan McCleary. "It tied in with all the stuff I wanted to teach."
Which is just what Alison Vaughan hoped it would do. "The idea is that the interest will carry on in the school afterwards," she says. It's hard to see her wish not being fulfilled in many more Manchester schools over the coming months. After all, as Des Coffey says, "there is nothing you can't teach through football, with a little imagination".