"Right, what were you doing last term?" he asked. "Sex!" came the aggressive answer, "OK," said Derek, unfazed. "Let's do sex." And they did. "Because I didn't know all the proper biological terms, I used the vernacular," he remembers. "By the end of the session, their jaws had all dropped open. "
Calling students' bluff is one of Derek's teaching methods. Another is magic - like making coins appear from his mouth. When he took 120 fifth-formers for an hour-long assembly, he used glove puppets and wind-up toys to communicate complicated ideas about philosophy. "There were a lot of wide boys there, but they loved it. It was one of the most wonderful hours I've ever had in teaching."
His third method is talk. "The easy thing to do is to give children 'busy' work: colouring, writing out, copying pages of sums. And there is a value to that. But it's most difficult to get kids to think and to make connections. You don't do that through the written word, the least useful way of communicating, but you can do it through the spoken word. So I use talking a lot, in a way they can understand but which isn't patronising."
He likes to tell stories, to use wit and humour - but not against the students. "It's the put-downs and sarcasms which they find hard to deal with. Then they'll respond verbally or physically."
And he prides himself on being open and honest, admitting when he doesn't have an answer, apologising if he's promised something he can't deliver. "Trust is the number one thing. You build that up by being straight with them; by telling them what you expect and by saying what you mean.
"It comes down to the fact that you're the manager of that group, so you have to decide what techniques work best. I look at the best examples of industrial management, which empower people rather than put people down. It means taking a risk, because you put a lot of yourself into it."
After a few years working in the oil industry and a spell of unemployment, Derek moved into teaching, learning "on the hoof". "What I brought into it was being an ordinary human being. I wasn't imbued with the idea of The Teacher, with status and power. I'm not scared to be myself. I'd worked in the outside world, on building sites, as a dustman, in the dole office - and I was on the dole. That was an education in its own right."
After 14 years at Earl Marshal, an inner city school where 70 per cent of the students are from ethnic minorities, his duties are multifarious. He teaches maths and science, is head of business studies, head of year, in charge of pastoral work and community liaison, and child protection officer.
Good relationships are vital. At the start of each year, he discusses course structures with students, and tells them what he hopes to achieve. They then write their own rules on how they want to work together. "You guide the discussion, gently but firmly - and kindly. Kindness sometimes gets written out of the school curriculum. Your job as a manager is to help them feel comfortable, to spot tensions and to make sure they don't build up."
Sometimes fights do break out. Recently, an ex-student - for no apparent reason - hit a fifth-former. Derek cuddled the victim to calm him down. "The whole area of touch is very contentious, but if we're wanting to communicate with them and get them to communicate positively, touching is important. Patting them on the back, giving them a hug, putting an arm round their shoulders, seems perfectly valid to me, if done within an atmosphere of trust."
He also tries to create an atmosphere of mutual respect. "One of my worries is that teachers get respect and fear mixed up and equate them. But they're not the same.
"I try to give some power to the students but, at the beginning, you have to establish authority. I've got two sheepdogs, and an old shepherd once told me that if the most dominant dog in the pack gets out of hand, you have to bite its ear, not hard, but enough for it to know who's boss."
He doesn't believe in "rewarding" errant behaviour with attention or punishment, but with patience, understanding, negotiation, even with silence. "You have to have a whole bag of techniques, and it almost becomes instinctive, which one to trawl out."
And how would he sum up his teaching style? "I've no idea. If I was taking a pedagogical theory: 'I'm a frustrated student-centred teacher who works within a didactic system.' A system where the bell and bureaucracy are joint tyrants.
"This sounds really old-fashioned," he says, "but I still think teaching is one of the noble professions, and teachers have been conned into forgetting that. We've got to have a pride in what we do."