The avalanche that swept over the group of 26 teenagers and nine adults is believed to have been triggered by people skiing off-piste further up the mountain.
The party, which included four accredited guides, was trekking near the ski resort of Les Orres, in the southern French Alps, near the Italian border, when disaster struck. Police said that the hikers were hit by a fast-moving, 1,000ft wall of snow. Many of the survivors were injured, six of them were seriously hurt.
The French Prime Minister, Lionel Jospin, travelled to the area last night, accompanied by two of his ministers. The President, Jacques Chirac, expressed his "profound emotion and great sadness" at the accident. Bernard Kouchner, the health minister, went to the youngsters' home town, just outside Paris, to meet their families.
More than 120 rescuers and paramilitary gendarmes, accompanied by search dogs, rushed to the scene after the alert was raised in the early afternoon. The rescue operation was hampered by the terrain, with a number of uprooted trees lying buried beneath the snow.
Bewildered youngsters, bundled in blankets, were placed on stretchers and ferried by helicopter to hospitals in the nearby towns of Embrun and Gap. French television showed anxious parents waiting at hospitals for news of their children.
As darkness fell, police said that there was little hope of finding anyone else alive under the powdery snow. But they planned to continue searching throughout the night, using chainsaws and long poles.
Weather forecasters had issued repeated avalanche warnings this week after recent storms laid a heavy and unstable blanket of snow in the region. Gerard Bouchet, a rescue team leader, said last night: "Personally, I wouldn't have gone trekking today in these conditions."
But there was confusion about whether the teenagers, who ranged in age from 14 to 16, had been walking off-piste. Initial reports said that they were snow-shoeing along the mountainside off marked trails above Les Orres. But local French sources later said that they were on an authorised route.
The group, who were from a high school in the middle-class town of Montigny- le-Bretonneux, were on a school holiday together.
Experts said that the risk of avalanches had been high all week, with more than three feet of snow having fallen in the Alps since last Sunday. The French youth and sports ministry has sent a team of investigators to the site.