The pink Post-its on the wall above Janet Willis’s desk flag up the usual headteacher preoccupations: links with a feeder school, dealing with a child’s special needs, what to do about the computers (again). And then you reach the last note: “Quidditch, Friday?” Janet Willis is head of education at Harry Potter. Not of wizarding education - that’s Albus Dumbledore - but real education. In the past year her school has had anything from three to 350 pupils - some soon to be among the most famous faces in the world. Her staff has ranged from two to 20, and classrooms have included Portakabins, marquees, caravans, hotel rooms and Durham cathedral library.

Mrs Willis was asked to teach when Warner Brothers started filming Harry Potter and the Philosopher’s Stone last autumn; a qualified languages teacher, she was offered a temporary film-tutoring job 25 years ago and has worked in films and theatre ever since. Within two days, she says, it was obvious that the sheer scale of the enterprise demanded the appointment of a full-time head. (Under English law, any child involved in film-making must have his or her school’s permission and approval from the local education authority, which carries out regular inspections. Children must receive between one and five hours’ tuition every day, to total 15 hours a week.) Driving across the Harry Potter studio site on a former airfield in Leavesden, Hertfordshire, you pass the backless suburban fascias of Privet Drive, where the story begins, before reaching a complex of ex-hangars and administrative buildings. Inside one of these are two floors of small, boxy offices, each containing a computer, three or four tables and chairs and a poster or two on the walls. One has a sink and some science clamps.

These are the Harry Potter classrooms, and seeing them brings home that this is no normal school. They are empty at the moment, awaiting the arrival of pupils acting in the second film, Harry Potter and the Chamber of Secrets, which starts filming this month. When full, they might contain 15 children spread across three or four rooms. Look into a Year 8 class, for example, and you might see a teacher plus gentle, clumsy Neville Longbottom, Harry’s horrible cousin, Dudley Dursley, exuberantly Irish Seamus Finnegan and the gruesome Crabbe, all in full make-up and costume.

Next door’s teacher might have charge of two uncannily similar Harry Potters, two Rons and two Hermiones - lookalikes picked to work on set as doubles while lighting, cameras and stunts are organised, allowing the stars time for their education.

The real Hermione, Ron and Harry - played by Emma Watson, Rupert Grint and Daniel Radcliffe - each have their own tutor and are generally taught in their own dressing rooms or trailers whenever they are not needed on set. Harry’s tutor, maths and science specialist Carmelina Wright, says: “I’m in Daniel’s room from nine to five. While he’s on set, I’m swotting up the subjects I don’t know. As soon as Daniel is free his father brings him to me and we start work. We get such a short time that you have to start teaching as soon as he comes in. It seems terribly hard but in fact it’s a good foil to the filming.”

The stars’ tutors teach a mix of their own work and work set by the child’s home school. The overriding principle, says Janet Willis, is that children miss nothing their peers do back in the classroom. What that means in practice, says humanities and art tutor Jane Ralley, is that every child works on whatever his or her school has sent. “You might have an 11-year-old doing the Battle of Hastings, a 14-year-old doing similes and metaphors in Shakespeare and a 10-year-old working on plurals in a story. You have to keep changing hats as you go round the room.”

This makes the Harry Potter school completely reliant on the children’s home schools to provide the right work, says Mrs Willis. She contacts each school before the actors arrive to find out about the child’s ability, level and syllabuses, and to discuss how the work is to be sent. Some schools fax it; others send it with the child. If the school has no spare textbook the film company will buy it; compared with other film schools, this one is richly resourced, with books, science and languages equipment and a laptop for every child.

Children come from primary and secondary schools (the youngest Year 5, the oldest Year 11), state and private, high-flying and bottom of the league. Most are hugely supportive, says Mrs Willis. A few are not. “If a child comes in with nothing at all we find something, but it’s difficult to motivate a child whose school shows no interest in the film.”

The children’s dressing rooms are along the corridor. This is where chaperones - mostly parents - who must legally accompany them everywhere apart from their lessons, read, drink coffee and play Scrabble. Through the window you can see the costume tent and, next to it, a large, empty space. Earlier this year it contained “Portakabin city”, says Mrs Willis: 30 temporary classrooms housing the 300 Hertfordshire schoolchildren who played Hogwarts pupils during the great hall scene.

They, too, had to be taught 15 hours a week for the three weeks they were on site. In addition, there were 40 local child extras on location in Durham, another 40 at Alnwick Castle and 20 at London Zoo. Mrs Willis hires a mix of supply teachers and other film tutors for these extras, and this time scoured the country to meet demand. “We used everyone we could lay our hands on for the great hall scenes. Grange Hill were furious.”

Teaching children in full Hogwarts costume and make-up becomes less exciting when you realise that they cannot do PE, and felt-tip pens are banned. But the greatest frustration, for tutors and children, is uncertainty. Film schedules change daily, sometimes hourly. Friday’s quidditch - requiring an extra tutor for the stunt doubles - will undoubtedly get switched to Thursday, or even next week.

Children constantly come and go with no notice, says Jane Ralley. Only half-hours of tuition count towards the weekly legal 15, so if a child is called back after 10 minutes in the classroom a tutor may try requesting another 20 minutes. But the film’s demands are all-consuming: “It’s very disheartening for children because they can never have a train of thought, never be sure of getting stuff finished. And when they come back it’s hard because you can’t expect a child to be foul over and over again on set, and then come into the classroom and immediately be charm itself.

“On the other hand, from a teaching point of view, this film is superb because the teachers in the story have powerful knowledge. They are respected - and that’s good for us.”