Possibly the world’s grimmest secondary-school building lies in a leafy suburb of the sprawling city of Phnom Penh, Cambodia. Tuol Sleng is an average-looking three-storey building following the model of many secondary schools in Asia: a block with a staircase at each end and covered balconies that pupils walk along to get from one classroom to the next.

In front of the L-shaped block is a grassy open area that would once have been filled with the voices of pupils taking their morning and afternoon breaks or doing PE or playing football or basketball. But that was before the Khmer Rouge took over in 1975.

An unnerving peacefulness hangs over the buildings of Tuol Sleng, or S-21 as it was called by the Khmer Rouge, for whom it was the main torture centre for the city’s people. Estimates vary on how many Cambodians were killed by Pol Pot’s regime, but 1.8 million - almost a third of the country’s population at that time - seems conservative. Of those, 17,000 passed through Tuol Sleng on their way to the killing fields of Choeung Ek just outside the city.

When the Vietnamese liberated Phnom Penh in 1975, just seven people of those 17,000 were alive. They found 14 decaying corpses lying on floors and the iron bedsteads on which they had been tortured. The Vietnamese buried them in the courtyard but left other things as they were. These rooms, with the grainy blow-ups of the corpses on the walls, are the first that you encounter.

For teachers, there is an added poignancy about the emotional turbulence that visitors to what was once Tuol Svay Prey High School undergo. These rooms, with bloodstains ingrained in the floor and the shackles that once held prisoners’ legs still attached to the beds, were once classrooms.

Their familiarity means you can almost still hear the chatter of students and the teacher’s voice and picture the rows of upturned faces. It is a stark contrast to the screams of the tortured that must have echoed around these bare walls in the late 1970s.

In the next building, things get worse. The Khmer Rouge documented its deeds extensively and with care. Tuol Sleng had its own photographer, whose job it was to take mugshots of all the prisoners.

These stark black-and-white pictures fill the next two classrooms in the adjacent school block. There are hundreds of them attached to rows of display boards. Many are of children, children who should have filled these classrooms with laughter. My eye was caught by one small boy of eight or nine, whose image would later haunt me as I visited the UN monument at Choeung Ek and looked at the heap of skulls carefully labelled by age.

Perhaps his was one of them. Perhaps his was still buried in the pits that have been left deliberately undisturbed.

Tuol Sleng makes any educator wonder what we are doing to prepare students for a world in which the deeds of evil men and women can and do thrive. How do you recognise evil, oppose it, deal with it, and even, like that little boy, suffer it? For me, Tuol Sleng’s silence was deep and heavy with reproach. The teaching of history has no finer advocate.