For Mr. Reichental it had started as bullying, with name calling at school escalating to physical abuse and ending with Bergen-Belsen. His message to the students is simple: “If you see someone being victimized, don’t be a bystander — stand up. If you see someone being treated badly, get involved.” Afterward, they gave him a standing ovation. Pupils and teachers then lined up to buy signed copies of his memoir, “I Was a Boy in Belsen.”

“Take this chance to ask me anything you like,” he told his young audience. “There are not so many of us left.”

It was mid-October 1944 when he was rounded up by the Gestapo in a shop in Bratislava, now the capital of Slovakia. For nearly 60 years, he never spoke about his experiences in Bergen-Belsen. He never even told his wife of more than 40 years or their three sons. But after her death in 2003 and his retirement from the Dublin jewelry business he owned, he has hardly stopped talking about what happened.

There may be an impression of making up for lost time, though he does not see it that way. “I owe it to the victims that their memory is not forgotten,” he said. “It’s not that I didn’t want to speak about it before. It’s just that I couldn’t. There are thousands like me; I believe it is nature’s way of allowing people to deal with things.”

His first classroom appearance, made at the urging of one of his sons, did not go well. He broke down as he told his story; worried parents would complain. But as he continued and his audience grew, so did his demand. Now he travels like some old-fashioned preacher to tell his story so younger generations will know what happened to him and to millions of others.