A disbelieving hush descends on the Home Counties sixth-formers touring the Auschwitz memorial and museum as they enter a room housing a mountainous display of human hair - equivalent to that shorn from 40,000 women, their Polish guide explains quietly.
The 183 students and two dozen teachers are halfway through a day trip organised by the Holocaust Educational Trust as part of its Lessons from Auschwitz project. On the preceding Sunday, they had gathered in London to hear an emotive address from survivor Susan Pollack, whose extended family was all but wiped out in the Holocaust.
In Oswiecim - the Polish town where the Nazis set up the death camps - the first stop had been the empty space once filled by the Great Synagogue. As students stand shivering in the snow, HET educator Julian Jeffrey passes around archive images of the road, formerly known as Jews' Street, to emphasise the harmonious pre-war relations enjoyed between Jews and local Catholics.
Before the Nazis came, Jews accounted for almost two-thirds of Oswiecim's then 12,000 population. Not a single Jew lives there now. Mr Jeffrey urges the students to visualise a thriving area of enormous spirituality and culture. "You need to see something you can't see."
Recognition of the former Jewish community is the nearby museum, incorporating a reconstructed synagogue. The building had survived by "a stroke of luck", points out London rabbi Barry Marcus, who regularly accompanies the HET groups. "The Nazis trashed everything inside it and used it as a storehouse." Explaining the significance of the synagogue and religious artefacts, he removes a scroll from the ark and invites the pupils, almost all non-Jewish, up to the bimah for a closer inspection.
The mood of the group becomes more introspective after the short coach journey to Auschwitz. Passing through the gateway bearing the callously deceitful motto "Arbeit Macht Frei" ("Work makes you free"), students and teachers take exterior photos of the grim barracks, watchtowers and barbed wire. Dan Facey of Lord Williams's School in Oxfordshire is filming for a 10-minute video on "what we can learn from history. We don't seem to learn the lessons."
As the local guide begins to explain the extent of the atrocities perpetrated - the gas chambers, executions, experiments, torture - the visitors are finding it difficult to fully comprehend the scope of inhumanity visited on Jews and other groups.
The students are told at the outset that people react in different ways. "It can be extremely varied," says Nicole Sarsby, an HET educator who teaches history at the Astley Cooper School in Hemel Hempstead. "Sometimes it's tears, sometimes it's asking lots of questions - a thirst for knowledge. Sometimes they are too overawed to speak. There is a sensory overload, particularly when it is cold."
Certainly the freezing conditions make the teenagers clad in multi-layers of winter apparel ponder how malnourished camp inmates clothed only in flimsy pyjamas could survive even a short spell of winter.
But when they move inside, the human tragedy really hits home. The exhibits of hair, shoes, spectacles; suitcases bearing the name and birth date of owners. A collection of shoe polish tins is a weirdly poignant acknowledgement of the geographical spread of victims. Matthew Pike, an HET educator and head of religious studies at Lord Williams's, observes that the display of baby clothes tends to resonate with the girls.
Visitors are alone with their thoughts as the litany of mass murder in the camps and beyond is relayed by the guide through their headsets. Well over a million deaths at Auschwitz-Birkenau, some 200,000 children among them.
Up to 80 per cent of new arrivals were sent to the gas chambers. Decisions of life or death were taken in seconds, as chillingly depicted in a photograph of the selection process.
There is general incredulity when the guide indicates the property on the site where camp commandant Rudolf Hoess - who was hanged at Auschwitz in 1947 - lived contentedly with his family. Indeed his wife referred to their home as "paradise".
Emma Cheshire from King Alfred's Community College in Oxfordshire reflects later that her overriding reflection from the day "is a real disgust in humanity. I wanted to find some understanding of what happened. I don't think I have. If you can't justify something, then how can you understand it?
"Our guide takes tours on a regular basis and she cannot grasp what caused it."
By the time the students reach Birkenau, the main killing centre, in mid-afternoon, the temperature has dropped further to the minus teens. The accompanying medic reports treating 10 people for minor ailments related to the weather and plans for an outdoor memorial ceremony for Shoah victims are abandoned.
First impressions are amazement at the sheer vastness of the site, the railway tracks on which victims were transported to their death jutting out in the snow.
"The emptiness is scary," says Rafael Llewellyn-Thomas from Beaumont School in St Albans. "You read about it in textbooks but you don't appreciate that it is on such a big scale. I'm not looking forward to tonight. What I've seen is going to turn over in my head."
As at Auschwitz, there are occasional stops for group members to read out relevant passages from the works of well-known survivors such as Primo Levi and Elie Wiesel.
Mr Jeffrey uses the story of Corfu Jews who endured a nine-day journey to the camp in appalling conditions to raise the question of whether the train drivers and those who organised the transport should be considered perpetrators or victims? And what about the bystanders who knew what was happening and did nothing to prevent it?
It is too cold outside to maintain proper focus on the words of the guide. But touring barracks adapted from stables built for 52 horses, the students pay rapt attention to her explanation of how life was for the 400-plus prisoners in the cramped, dank area.
The memorial service is moved indoors, appositely close to displays of victims' stories, accompanied by montages of smiling family photos. "The day has been about humanising victims and it puts a face to them," Dan Facey says.
Leading the prayers, Rabbi Marcus notes the relevance of Psalm 23 (Psalm of David) because of the passage: "Though I walk through the valley of the shadow of death, I will fear no evil." He also recites the Hebrew prayer for victims, El Male Rachamim.
He tells the students that what happened is beyond comprehension. For the young and strong, the average lifespan was six months "in this godforsaken place". Putting the genocide into a contemporary context, he likens it to "a 9/11 for 500 consecutive days".
The victims "were part of communities, schools. They had dreams, all taken away from them because of the faith they were born into."
With rising antisemitism around the globe, tackling prejudice is imperative, he adds. "Preaching hate is never, ever harmless."
Before sounding the shofar - "a symbol of hope for the future" - he urges students to leave with humility and to undertake not to be fearful of those who look different or worship in a different manner.
The pupils' final act at Birkenau is to light memorial candles at the camp entrance.
During the one-hour coach journey back to Krakow Airport, King Alfred's student Ed Preece - who is reading historian Sir Ian Kershaw's Hitler biography - takes up a point made by Rabbi Marcus about the threat posed by those who disseminate Holocaust denial material. "The best thing is to debate people like David Irving and expose the flaws in their argument," he says.
"The next few years are pivotal for getting the message of the Holocaust across while survivors are still alive. Susan Pollack lost 50 members of her family. I had real respect for her for speaking up."
Processing his thoughts of the day would take time. "You are taking in so much at once, it's unbelievable." Experiencing the wintry conditions, even for a short time, "greatly increased my emapthy with the victims in a way that no book or film can".
Jess Angell from Henley College will be returning to Auschwitz in April as part of a school trip and feels that doing the HET programme "will help me to cope with it better. She also ponders the fact that "we are the last generation to hear from survivors. What is going to happen after they go?
"A lot of young people today are in a state of apathy. It takes the message from something like this to shake them out of it."
The latest group takes participation in Lessons from Auschwitz beyond the 15,000 mark. The final elements of the programme - which is supported by the government and the Welsh and Scottish administrations - are a post-trip seminar and taking what they have learned into their schools and beyond.
Dan Facey and Rosie Pringle were chosen to represent Lord Williams's from 70 applicants, each of whom had to submit a 250-word essay on why they wanted to go and what they would contribute on their return.
Every year, survivor Ziggy Shipper speaks at the school, where Mr Pike knows of five Jewish pupils among a roll of 2,200.
He attributes his own interest in the subject "to my having a great teacher as a kid who was passionate about the Holocaust - she was awesome. I try to kindle that interest in my students."