As found in 1979
The five former high school buildings of the complex were converted in 1975 into a prison and interrogation centre. The Khmer Rouge renamed the complex "Security Prison 21" (S-21) and construction began to adapt the prison to the inmates: the buildings were enclosed in electrified barbed wire, the classrooms converted into tiny prison and torture chambers and all the windows were covered with iron bars and barbed wire to prevent prisoner escapes.
From 1975 to 1979, an estimated 17,000 people were imprisoned at Tuol Sleng (some estimates suggest a number as high as 20,000, though the real number is unknown). The prisoners were selected from all around the country, and usually were former Khmer Rouge members and soldiers, accused of treason.
Upon arrival at the prison, prisoners were photographed and required to give complete biographical information. After that, they were forced to strip naked, and 
all their possessions were removed. The prisoners were then taken to their cells. Those taken to the smaller cells were shackled to the walls. Those who were held in the large mass cells were collectively shackled to long pieces of iron bar. The prisoners had to sleep on the floors, while still shackled.
After the interrogation, the prisoner and his/her family were taken to the Choeung Ek (the Killing fields) extermination centre, fifteen kilometres from Phnom Penh. There, they were killed by being battered with iron bars, pickaxes, machetes and
many other makeshift weapons. Victims of the Khmer Rouge were seldom shot as bullets were viewed as too precious for this purpose. Walking round is extremely harrowing and literally bar from a clean up and photographs of those incarcerated in some rooms everything is as it was when it was "liberated" by the Vietnamese in 1979. When the Vietnamese arrived at Toul Sleng they found that the guards had hurriedly left and 14 rotting corpses remained on the beds on which they were tortured to death for information they probably didn't have.
The bodies were buried in a remembrance garden on the grounds. It is hard to come to terms while there that all the pictures you see are of people tortured to death, or sent to the Killing fields, only 7 people were found alive in the complex in 1979, out of 17,000 - 20,000