At 2.45pm on Monday March 22 last year, the skipper of a fishing boat, Spanish Eyes, spotted a red kayak bobbing two miles south-east of Lyme Regis. He immediately radioed the coastguard at Portland - a message that was the first firm evidence of a tragedy that day off the Dorset coast.

Nearly four hours later, Britain’s worst canoeing disaster had unfolded as the last survivors - four sixth-formers, a teacher and two instructors - were winched aboard rescue helicopters. But by then four others - Dean Sayer, 17, and 16-year-olds Claire Langley, Simon Dunne and Rachel Walker - were dead. Despite last week’s convictions for manslaughter, what makes their deaths so unbearable for their families and classmates is the knowledge that they could have been saved if any one of a dozen simple errors by the instructors, the centre, the company and the coastguards had been avoided.

Most of the party from Southway school had never sat in a canoe before. They practised briefly in the swimming pool of the St Alban’s outdoor centre, the day before they arrived at Town Beach at 10.15am to paddle two miles across Lyme Bay to Charmouth and back. The trip, on the second day of a week’s stay, was due to take less than two hours.

Trouble began immediately. Dean Sayer capsized while still close enough to the shore to stand up. Tony Mann, the 23-year-old instructor, asked if he wanted to go back but Dean said he could make it. Teacher Norman Pointer rolled over five times within minutes, and was violently sick. Mr Mann considered returning then. But as he tried to help the teacher for the final time, he looked up.

Where previously, 30 yards away, had been the nine other members of the party, there was now just sea. Formed into a raft for stability against the head-high waves and free of the “wind shadow” of the cliffs, the canoes were being blown out to sea.

Within 20 minutes, despite the efforts of the other instructor Karen Gardner, all but one canoe overturned and began to sink, leaving the pupils clinging to the last craft in seas 9C above freezing. They were dressed in swimming costumes and wetsuits, but without gloves or headgear. The youngsters asked if they should inflate their life-jackets, but Miss Gardner, 21, said no, feeling sure rescue would come when they failed to arrive on time. To stave off panic, the children held hands, recited prayers and sang songs.

The centre’s handyman was due to meet the party at Charmouth. At 12.25pm he reported the children missing to Joseph Stoddart, the St Alban’s manager who served 22 years in the Army and was an expert canoeist. Mr Stoddart searched the shoreline for half an hour in a rescue boat and then drove along the shore looking for canoes. But it was not until 3.07pm that he told the coastguard. He said the instructors were equipped with flares and well qualified, but in fact they were barely able to make the crossing themselves.

Seven minutes before his call was made, the last canoe sank and the children were left helpless, their life-jackets becoming waterlogged. In desperation, Samantha Stansby and Emma Hartley began to swim towards the coast for help. Others slipped into unconsciousness from hypothermia and fatigue. When found, after delays and errors by the coastguard, they were up to eight miles from their intended destination.

The disaster was no surprise to former St Alban’s instructors Joy Cawthorne and Richard Retallick. They wrote to Peter Kite, the managing director of Active Learning and Leisure which owned the centre, nine months before the deaths. They said: “We think you should take a careful look at your standards of safety, otherwise you may find yourself trying to explain why someone’s son or daughter is not coming home.”

They claimed safety procedures were non-existent; there were no flares, tow-ropes, waterproofs or first-aid kits provided and there were too many children per instructor. The couple resigned after five weeks. As Mr Mr Retallick packed the car to leave, he warned Mr Stoddart again about safety. The manager replied: “It isn’t going to happen here.”