I had been for three years head of the physics department of the East Middlesex Comprehensive, with four masters and two technicians within my domain. My future, from my present age of thirty-three, looked like a possible deputy head-mastership, most likely with a move involved, and even perhaps a headship, though if I hadn't achieved that by forty I could forget it. Headmasters got younger every year; mostly, cynics suggested, because the younger the man they appointed, the more the authorities could boss him about.

I was, all in all, contented with my job and hopeful of my prospects. It was only at home that things weren't so good.

Four A learned about momentum and Arcady ate his apple when he thought I wasn't looking. My peripheral vision after ten years of teaching was, however, so acute that at times they thought I could literally see out of the back of my head. It did no harm: it made control easier.

'Don't drop the core on the floor, Paul,' I said mildly. It was one thing to let him eat the apple – he'd deserved it – but quite another to let him think I hadn't seen. Keeping a grip on the monsters was a perpetual psychological game, but also priority number one. I'd seen stronger men than myself reduced to nervous breakdowns by the hunting-pack instincts of children.

When the end-of-lesson bell rang, they did me the ultimate courtesy of letting me finish what I was saying before erupting into the going-home stampede. It was, after all, the last lesson on Friday – and God be thanked for weekends.

I made my way slowly round the four physics laboratories and the two equipment rooms, checking that everything was in order. The two technicians, Louisa and David, were dismantling and putting away all apparatus not needed on Monday, picking Five E's efforts at radio-circuitry to pieces and returning the batteries, clips, bases and transistors to the countless racks and drawers in the equipment rooms.