Breaktime at Clifford Road Primary in Ipswich, and the playground is filled with running, jumping and skipping children. A foot below the asphalt, air raid drill is in full progress and a tube train is bumping and rattling its way between stations.
Hundreds of older British schools must have air-raid shelters on site. But few will have transformed the tunnels into an interactive Blitz museum so realistic that one elderly visitor dived for cover when she heard a Doodlebug apparently cut out overhead.
Headmaster Richard Cove realised the educational potential of the shelter when he and a group of energetic parents accidentally broke into the entrance while digging a pond for nature studies.
But it would probably have remained no more that an intriguing quadrangle of tunnels without the enthusiasm and technical know-how of Fred Grigg, an expert in special effects and World War Two who has now become so much part of the school that he has been co-opted on to the governing body. His own five-year-old daughter is now a pupil.
"For children working on life in Britain during 1939-45, it's of real interest being able to come down here. This is far better than a book or a video. This is the actual thing," says Mr Cove. "Children have never been down a shelter, and when they do amazement is the first reaction. It gives them a fair idea of what life was like for children then - they are a bit more respectful of that."
Once Fred Grigg - dressed as an air raid protection warden - has finished taking his young charges round the shelter, it's hardly surprising that they have learned a little more respect for the everyday hardships of their grandparents.
Clifford Street pupils get their first introduction to the shelter in authentic fashion - when the air-raid siren goes off unexpectedly during a lesson. During the war, the school's 600 pupils evacuated to the three playground shelters in just two-and-a-half minutes.
Once underground, they are ushered into a gloomy tunnel with benches along one wall. There, a concealed - but extremely powerful - stereo system blasts and booms out the frighteningly authentic sounds of a raid, recorded by Rank and the BBC in 1940. The mournful wail of the sirens is followed by the sound of German planes ("Heinkels," explains Mr Grigg helpfully) and the thud and rumble of bombs falling.
Perhaps most eerie of all are the bells of the fire engines racing round overhead and then the tell-tale sound of the feared Doodlebug - the flying bomb which could cut out and fall at any time. The whole experience sends shivers down spines.
"It has reduced older people to tears," says Mr Cove. "We wanted to impress on the children that once the raid finished and they could leave, they wouldn't know if they were going to go back to a house or not."
Reinforcing that message is the bombed street which forms the next part of the shelter, complete with authentic rubble removed from the shelter's blocked staircases and cheery "business as usual" notices on shattered shop windows. Here, too, Mr Grigg has plans to recreate the realities of life in war-time; for instance, he knows where he can lay hands on a dentist's drill, pedal-operated and the kind that was commonly used when emergency cases of toothache required an impromptu mobile surgery.
Perhaps the most extraordinary part of the shelter comes next - the walk on a London Transport-style escalator into a miniature Tube station, with a real train interior just around the corner. Since many Suffolk children have never travelled on the Underground, this part of the museum is a valuable experience of how thousands of Londoners moved around and sheltered during the Blitz.
The tunnel may be smaller than the real thing, but the sound-effects of trains arriving and leaving lull visitors into a false sense of extreme insecurity.
One BBC cameraman, filming almost on the track, took several hasty steps back when warned jokingly that a Tube was about to pull into the station.
And then there is the pi ce de resistance: the Tube train interior, cunningly cannibalised from bits of just post-war train and other odds and ends, carefully miniaturised to fit the shelter tunnels. Again, sound- effects recorded on a real train complete the illusion, with lights timed to flicker when the "train" goes over the points. One teacher even fell forwards when the train "stopped" - even though it doesn't actually move.
Mr Cove believes that but for Fred Grigg's expertise and time - he had to give up work after breaking his neck in an accident - the shelter would still be "a hole in the ground". It is his technical knowledge which has fulfilled the requirements of health and safety inspectors, and turned electrical bits and bobs into special effects on a financial shoestring, now augmented by frequent paying visits from other schools anxious to bring to life the requirements of key stage 2 and 3 in history (Home Front 39-45).
But for imaginative children, much of the lingering memory of the experience comes in the wartime exhibits and Fred's encyclopaedic and gossipy knowledge. A natural storyteller and showman, he instinctively knows which details will appeal most to children of different ages - and adults, too.
One of the prize exhibits is the small collection of fake wartime soap, as sold by spivs. There is the Kilty soap, generally sold with the confiding promise that the buyer could sell it on at a profit. But more than once recipients discovered its principal ingredients were lard and talcum powder.
Another is the Jester saltwater soap, which looked deceptively like the household brand of Sunlight once the name had been removed with a hot spoon.
Mr Grigg explains: "It washes in salt water. The only problem is that it doesn't work with ordinary water, and if you get it on your skin you smell like a rancid polecat. I know, because I made the mistake of touching it and couldn't get rid of the smell for days."
There is an authentic, razor-sharp, bayonet, a selection of Home Guard literature on improbable methods of street-to-street fighting to be used in case of invasion, gas masks, and, what Mr Grigg endearingly calls the thunderbox.
This convenience - one of just five originally in the shelter - consists of a wooden shelf with a round hole cut in the middle, a bucket ("the original") beneath, and a curtain.
"They had to hold it shut with a foot, whistle or sing very loudly - and be quick. Can you imagine it with all those children down here?" Clifford Street pupils can. It is one of the things that makes them shudder about wartime life, along with lack of meat and living on delicacies such as powdered egg and the vegetarian Lord Woolton's Pie.
"You'd have to sing and whistle. It would be so embarrassing," muses nine-year-old Emily Boyd.
Astonishingly, out of a selection of six Year 5 pupils, only one admitted to having known anything about World War Two before - surely some must have watched a repeat of Dad's Army without having made the connection?
"It's not a bit like Dad's Army," says Nathan Meech, 9. David Traynor, 10, told me: "There was a programme on telly about it but I can't remember its name. Learning about the war was a bit of a surprise."
One of the few plus points the pupils could see was the potential for play. Liquorice, apparently, was a vital ingredient for cowboys and indians. Well licked, it could be used to make markings on the face, while gobstoppers also had their uses.
One of Fred Grigg's proudest possessions is the scrapbook containing letters from pupils from all over the county who have visited the shelter as part of their work on the Home Front. Reading some of the comments demonstrates how their tour brought the reality of war home to children.
"I like the noise which made my tummy rumble and it was loud, it made me block my ears," wrote Tom.
"I liked it when the lights went dim and then we heard the bombing. I felt as if I was really there. Now I know how the children felt in those days," wrote another. "I especially liked hearing what happened with the soap. Love Jade."
There is a darker side. At least one school shelter received a direct hit after a bomb skidded across the playground and down its mouth. The Bloomsbury shelter in central London was sealed as a war grave. News of another hit on a school shelter in Croydon was suppressed by the Government for reasons of morale. And communal shelters all over the capital suffered, including at least one Tube station.
There were 88 raids on Ipswich - an easy target on a river estuary - and at least 200 local people slept in the Clifford Road shelters every night, overseen by a large fisherman's wife who kept order and the place spotless for 46d a month. The worst damage to the school came from odd bits of machine-gun fire to the roof, probably by air crews using up ammunition before turning for home. The damage was only recently discovered when the tiles were replaced.
But there is one secret the adults are keeping. George, the shelter ghost. Many of the parents who put in unpaid hours labouring in tunnels have heard footsteps. Brooms and keys move. Lights switch themselves on overnight.
Mr Grigg is affectionate towards his spook. "No one died down here, we've checked in the school logbook. He's just a memory. But if we've got a party coming, we say 'George, behave yourself. And he does'."
Clifford Road Primary School, Clifford Road, Ipswich PIP4 1PJ. Tel: 0473 251605.