Few teachers could claim they have never thought of escaping from the classroom. But for Andre Heintz, teaching in Caen in Normandy in the late spring of 1944, an escape plan was not an idle daydream but a matter of life and death.

He knew that if the Gestapo found out he was working for the Resistance, they might come for him at any time. And the buildings to which his lycee had been evacuated were traps. One was a former station with a spiral staircase; the other an old orphanage with an enclosed garden behind. “I would have been shot like a rabbit in the kitchen garden,” he says.

So when the school caretaker came into his English class one day early in May and told him a Madame Bergeot urgently needed to speak to him, M Heintz was instantly on the alert. Bergeot was the real name of his intelligence contact. For security’s sake, he was always known as Courtois. Was this a Gestapo trap?

He left his class of boys to amuse themselves, hurried down to the main entrance, took the woman by the arm and led her out into the street. He wanted to check there was no Gestapo car lurking. Seeing nothing suspicious, he took her back inside. She then told him six phrases to listen out for on his radio that would give him vital information about a forthcoming invasion: D-Day. He learned them by heart. But not until some weeks later, crouched over the secret crystal radio in his family cellar, did he hear the phrase announcing that the Allied invasion was imminent.

“L’heure du combat viendra,” came the sign-off at the end of the BBC news on June 1, 1944. “The hour of combat is coming.” He knew he must alert his contacts to stand by to rise up in support of the Allies.

His story is told in Ten Days to D-Day by the historian David Stafford, which reveals the human side of the run-up to the most decisive invasion of the Second World War. It began the advance eastward that ended in Hitler’s suicide and the surrender of all German forces within less than a year.

Neither M Heintz nor anyone else outside a tiny circle at the top of Allied command knew the main invasion was to take place on the nearby Normandy beaches. A careful campaign of disinformation persuaded the Germans it would be further north, where the Channel is at its narrowest. Nor did they know exactly when it would happen. Only the Allied leaders knew the provisional date of June 5.

But that changed at the last moment, when the Allied Supreme Commander, General Eisenhower, took the nerve-racking decision to postpone the plan by 24 hours because of bad weather. Not until early on the morning of June 6 did the sound of planes and heavy anti-aircraft fire alert M Heintz to the fact that the moment had come and the place was Normandy. He advised his mother to fill the bath and cook some food before the water and gas were cut off. He spent the rest of the day helping the Red Cross cope with casualties. He and his sister stopped the Allies bombing the hospital where she worked by laying blood-soaked sheets on the roof in the shape of a cross.

The Allies landed 156,000 troops on the Normandy beaches on June 6, but it took another month of heavy fighting before British and Canadian forces entered Caen, on July 9. By August 25, when the battle for Normandy was won, and Allied forces entered Paris, more than 425,000 troops on both sides had been killed or wounded, including 53,000 Allied dead. Between 15,000 and 20,000 French civilians had also lost their lives, most as a result of Allied bombing.

Now 84, Andre Heintz is still recognisably the trim figure who cycled round the lanes near Caen, gathering information about German troop movements and scouting for hiding places and clear fields for parachute drops. He could also gather intelligence closer to home; his family lived just 100 yards from the local German military headquarters. To reach home, they had to pass through a barrier and show their identity documents. Such restrictions infuriated the young Andre, who, even before the war, already had reason to hate the Germans. His father’s parents had left their native Alsace after Germany annexed the province following the Franco-Prussian war of 1870-71.

Then his father, a classics teacher, had been driven from his home in Burgundy by German troops during the First World War and had settled in Normandy. Now they faced occupation once again. The young teacher was determined to do what he could to end it.

Andre Heintz was an Anglophile, having spent six months at Bristol grammar school when he was 15. (He loved the sport and - by French standards - the relaxed academic life.) He resolved to join the Resistance and help the Allies. He was initiated into Resistance work in the autumn of 1940 through a Franco-Polish network set up by Father Makulec, a priest working with Polish immigrants in Normandy. He helped find out where the Germans were concealing some of the bombers used in the Blitz on London.

Then he started to help with forging French identity cards, sometimes for the few Jews in the area but more often for US servicemen shot down (they had to pretend to be deaf and dumb) or for French underground workers.

After that, he became involved with collecting and passing on information.

Andre Heintz is one of 10 ordinary people whose stories of life in extraordinary circumstances are told in Ten Days to D-Day. Based on letters and diaries of those involved, it describes their hopes and expectations - but also the real dread of failure.

“Most of the histories about D-Day cover the landings and the Normandy campaign,” Dr Stafford explains. “There was no point in doing that again. I wanted to write a book that would be accessible to those who wouldn’t usually read military history.”

His cast includes Veronica Owen, a 19-year-old Wren (and later headmistress of Malvern girls’ college) working on codes and signals near Portsmouth; Albert Grunberg, a middle-aged Jew concealed from the Nazis in a Paris garret; and Walter Schwender, a young German soldier stationed in northern France. The accounts of their daily lives are interwoven with those of the leaders on both sides: Eisenhower, Churchill, Hitler and Rommel.

We read of growing tensions as the invasion approaches. Churchill, maddened by his lack of direct involvement, proposed to accompany one of the ships to Normandy with King George VI. Eisenhower had to intervene to suggest to the King that he must persuade Churchill not to go.

After the war, M Heintz escaped the devastation of Caen to spend two years as an instructor in French at the University of Edinburgh. For the next 36 years, he taught French as a foreign language at the University of Caen’s institute of technology.

Now long retired from teaching, he has five children, 12 grandchildren and a farmhouse he is busy restoring 80 kilometres from Caen. But he still seems to spend most of his time reliving those momentous events of 60 years ago. He gives lectures at the city’s impressive Musee Memorial and leads guided tours of the museum (stopping, of course, to point out the spinach tin that housed his old crystal radio) and the landing beaches. He is also in constant touch with historians.

“He is definitely a local hero,” says Michael Durham, a TES contributor, a former pupil of Bristol grammar and the French exchange partner of Andre’s son, Michel. “He was treated with awe and respect.”

Mr Durham recalls how, as he drove the boys around the lanes of Normandy during the exchange visit, M Heintz might point out a barn and say: “I remember standing there waiting for the Boche one night.” They once found the remains of a bullet-scarred US jeep. “He may spend a lot of time reliving the past,” says Michael, “but he is very accessible to the younger generation and easy to talk to. And he still has extraordinary energy and passionate commitment.”

Does he ever take German groups around the beaches? “I avoid it,” M Heintz says simply. But he has done his utmost to ensure that his children do not share his feelings about the Germans, and he recognises that Germans have been among his best students.

He is not sure that people learn from history, even though he spends so much of his time teaching it. But he says it is vital that teachers do what they can to discourage hatred. “Hatred is an emotion that is difficult to root out,” he says. “And all it does is grow.”

Ten Days to D-Day by David Stafford is available in paperback (Abacus, pound;7.99) and will form the basis of a Channel 4 documentary to be shown in two one - hour parts at 8pm on Monday 24 and Tuesday 25 May, and repeated in its entirety at 8pm on Saturday May 29