It's 9am and class 7:1 are engrossed in a geography assignment about the area where they live. A small, bespectacled boy at the front puts up his hand. "Sir, you know how you said we should describe the streets round our house? Well, I live on a kind of island. So there aren't many streets where I live."
Doug Harrison shrugs. Ilford in east London is not generally known to be part of an archipelago, so it's difficult to know how to respond. He has a magnificent shrug, though. His shoulders go up and his wrists splay outwards, palms facing the ceiling. In extreme situations his arms spread out like Antony Gormley's Angel of the North sculpture. His lived-in face takes on a comical expression. Class 7:1 giggle. Pupils like Mr Harrison.
Reassured that they can, if they wish, describe parks and other non-street features, the children settle back down to work.
Teaching geography is only one of Mr Harrison's jobs. He's Seven Kings high school's pastoral head and head of Year 10, so his day incorporates an enormous range of tasks. His favourites are those that involve talking to pupils. This morning, though, he has other things to do. He needs to see the head about some staffing issues. A list must be compiled of Year 10's regular latecomers and poor attenders so action can be taken if necessary.
And he needs to check up on a boy whose mother has been warned she faces court if his attendance doesn't hit 100 per cent. It hasn't; he isn't here.
Someone has phoned to say he'll arrive at lunchtime.
Morning break and Mr Harrison is in one of his regular habitats, policing the queues outside the canteen. "That's just what we need. One of the tills has broken... Packed lunches? OK, on you go... Yes, supported Everton man and boy; since Rooney's gone we're up to third in the league... Kylie, I'm not sure we should be wearing quite so many badges... Is it the 21st? My wedding anniversary tomorrow. I shouldn't have forgotten that."
The canteen is fairly quiet today because it's Ramadan and the school's Muslim pupils are fasting. So he has time to draw breath. He's seen a lot of changes since he arrived in the London borough of Redbridge as a newly qualified teacher 35 years ago. His first job was at Downshall secondary modern. When it merged with Beal grammar school for girls in 1975 to form Seven Kings high, he moved over. There were a couple of difficult years, he says. Some of the grammar school staff didn't adapt well to mixed-ability teaching, and the Downshall boys couldn't believe their luck at finding themselves in a mixed-sex school. But there was always a certain structure to the place. Downshall was a well-disciplined school and a nucleus of experienced staff who stayed through the next three decades have helped maintain that.
At 11.05am the bell rings and Mr Harrison plunges into a Year 8 lesson on open-cast mining. This class held its own public inquiry last week on whether "Wearham" should have a mine. They voted against; they usually, do Mr Harrison reflects, and now they're writing up the exercise as a report for the Wearham Gazette. A dishevelled boy flies in from the computer room, where he's been working independently, and registers a disapproving look on his teacher's face. He tries to launch into an explanation of his recent movements but stutters to a halt. Mr Harrison has snapped into disciplinary mode. "I'm looking at the state of you. Tuck your shirt in. And your tie."
Chastened, the boy tidies himself.
Lesson over, Mr Harrison has pastoral issues to deal with. He heads for the medical centre to discuss a vulnerable boy who's not coping well with school. The trouble is there's nowhere for him to go. The local psychiatric unit takes only severe cases and there's no pupil referral unit for under-14s in the borough. The school is hoping for some home tuition, but even that won't be ideal. It's a serious problem. Some pupils need a small-scale, caring environment where they can feel safe. But no such place exists.
There is some new hope for Seven Kings' more problematic pupils. This afternoon, Mr Harrison has an appointment to visit one of three Year 11 boys who are on full-time courses at Redbridge college; Seven Kings hopes a wider range of students will take advantage of its facilities in the future. Before he leaves he checks again on the missing boy. Still no sign.
His mother will be in court in a couple of weeks.
Today at Redbridge he's seeing a student who's doing well and attending college regularly but whose homework assignments are causing concern. Is the boy keeping up with his work? The student embarks on a genial explanation of a recent misdemeanour. "Well yes," he begins, "but you see my presentation that I was doing on Monday? My sister got hold of the disc and, well, she pressed the wrong button. Blip! That was it, Sir. Gone."
Mr Harrison finds it difficult to keep a straight face and rearranges his features into a world-weary expression. "I've heard it all a million times," he says. "And what's more, you know I have."
There's nothing world-weary about Mr Harrison, though. "When people ask why I've been at the same school for 35 years," he says, "I tell them I wake up in the morning and I'm happy to go to work."