Working in pairs, they embark on carving a human head from one of these blocks. The dominant sound is the "tap, tap, tap" of hammers on chisels. "The head is in there somewhere," says Mo Gardner. "You just have to find it, to let it out."
During the course of the day, that is exactly what happens: as the result of an enthusiastic team effort, four splendid heads emerge from the dull, grey concrete.
Heads may seem an over-ambitious project for children who have never held a hammer and chisel before, but they get used to the tools impressively fast. These are not portrait heads; they are totem poles and gargoyles.
Mo always gets beginners to start with a head. "After all, everyone's got one," she says. "And a cube-like block is perfect. The best way to get a nose is along one of the long edges, by leaving the edge prominent and cutting away on either side for the cheeks and eyes, and underneath for the upper lip."
The whole class of more than 30 children are taking it in turns to carve the four heads. The next group of eight will shape the bulge of the eyes in their sockets, the next will do the mouth - and the final group will work on ears, chin, headdress, perhaps teeth and tongue or even hands held to the face. As voices pipe up with questions, Mo administers sound advice: "I don't mind if you hit your thumb, but your mother might - and keep your goggles on!" She holds a young hand steady, or smooths a rough patch of cement that is proving stubborn.
The children have already heard Mo's introductory talk. She has emphasised the importance of goggles; she has introduced them to the hammers, the different chisels, dummy mallets, stone rasps and rifflers (a kind of file), adding that they are her tools and she expects them to be treated properly. Finally, she says that there is to be no trouble. There never is. "I don't take prisoners," she warns them.
The children show impressive concentration. Their class teacher, Kevin Ryder, is amused to see how their characters emerge in their work, and Sarah Guthrie, the school's arts co-ordinator who has organised Mo's visits, is delighted that some of the best work is done by children who are not strong in academic subjects.
The day is punctuated by visits from teachers and parents. At some of the schools where Mo teaches, extra classes have had to be arranged so that other staff and parents, and even governors, can have a go. She hopes that there will be someone who will carry on with the work she has started.
She is quite clear about her objectives for her pupils. "All the children will have a sense of achievement. None of them will 'fail'. And they should not be bored." She adds that her principal aim is "to demystify sculpture as an art form, and then, when they aren't so scared of it, they might eventually learn to buy it as readily as they buy paintings.
And, of course, enjoy doing it themselves. "
Now in her early fifties, Mo Gardner was originally trained in window display (ending up in Liberty's of Regent Street, London), moved into antiques, and then worked in her family's jewellery business. She moved gradually into sculpting, after the birth of her two sons, now 27 and 25.
Since then she has done part-time freelance teaching in schools and adult education colleges. She works a lot on commission, has demonstrated in Hungary and Scotland, and this Easter - in spite of having one leg in plaster after falling into an unguarded manhole - was teaching Israeli and Palestinian children together in Jerusalem, an initiative for which she raised the money herself.
Celcon, the breeze-block makers, gave her a lorry-load free, and British Airways flew her tools out. Her elder son Paul acted as wheelchair pusher and video recordist. She has also found time to become a therapist and healer.
She is spending eight days in Nascot Wood, teaching each age group. The work of the youngest class is already on display in a small internal garden in the main building. This particular residency was born in sad circumstances: a much-loved teacher, Mrs Elizabeth Thorpe, died last year in a skiing accident, and the money raised in her memory was given by her husband to the school.
Staff, governors and Mr Thorpe together decided to spend it on a project that would involve everyone. Part of it was spent on a school memorial garden tended by the pupils, but the staff already knew of Mo's work. Two years ago she taught at Cassiobury infants' school nearby, where 13 Celcon-block heads (there were once 15: two were stolen) were carved by the five and six-year-old pupils who also made a seven-foot totem pole.
With the agreement of Mr Thorpe and the governors, the Nascot Wood staff decided that #163;800 should be spent on Mo's eight-day residency. Sculpture also fitted in with the national curriculum requirements and everyone felt chiselling made a change from clay modelling.
The children have certainly revelled in the experience and, from the enthusiastic show of hands in class later, clearly demonstrated that they would all like to have another go.
Not many teachers of primary children would have parents and teachers, even governors, clamouring to attend school. But that frequently happens when Mo Gardner is in residence. When she taught a class at the Francis Holland School for Girls in Chelsea last winter, the headmistress, Jennifer Anderson, had to arrange a special out-of-term day for her staff to have a turn.
Mo Gardner charges #163;100 a day and brings all her own tools and goggles. She is at Cluny Cottage, 44 Cravells Road, Harpenden, Herts AL5 1BD. Tel: 01582 461046, fax: 01582 620477.
u She also does portraits and other sculptures to order. Her adult education classes are at Oakland College, Harpenden, and her work is on display at Stockwood Park Museum, Luton