Revelstoke, B.C. — Seven teens on a high-school ski excursion in the treacherous back country of east-central B.C. died Saturday when a half-kilometre-wide avalanche roared down from a mountainside and engulfed their group.

It was the second deadly slide to hit the region in as many weeks. Seven adults perished in a slide in January.

The dead were six boys and one girl, all in Grade 10 at Strathcona-Tweedsmuir School, a private day school on an expansive rural campus in the foothills southwest of Calgary.

"We are absolutely stricken with grief. Our hearts go out to all those who are impacted," Tony Macoun, head of the school, said in a written statement.

Frantic parents gathered at the school Saturday night, arriving separately and in groups, embracing each other. No students were visible.

"Our school is a small family and loss this extreme is impacting us tremendously," said Mr. Macoun.

There were 17 people in the school group, three adults and 14 Grade 10 students in an outdoor education class, on their annual cross-country ski trip.

The avalanche occurred on Mount Cheops in Glacier National Park just before noon, said Pam Doyle, superintendent for Parks Canada in Revelstoke.

It happened on the north face of the mountain in Connaught Creek Valley section of Balu Valley, about five kilometres west of the Rogers Pass summit.

The group was led by two male teachers and a male volunteer. All had certification and back-country experience, Mr. Macoun said.

The group had been skiing about 15 metres apart, with one supervisor in front and the other two bringing up the rear.

The skiers were halfway up the valley when the avalanche roared down the north slope, burying nearly the entire group.

Another group of young skiers witnessed the avalanche and were among the first to help.

A supervisor whose hand was just visible above the snow was the first to be dug out. Once freed, he called park wardens on a satellite phone to raise the alarm.

"Of the remaining students, six of the students are safe and one has a broken ankle," Mr. Macoun said in his statement.

The three leaders were uninjured and are attending to the survivors, who were in Revelstoke on Saturday night after being airlifted first to Glacier Park Lodge by helicopter.

Two people from the group were treated for minor injuries, said RCMP Sergeant Randy Brown.

The avalanche was "very powerful," with a rating from 3 to 3.5, compared with a top rating of 4, said Pat Dunn, a Parks Canada spokeswoman.

"The size 3 to 3.5 avalanche could destroy a building . . . or 10 acres of forest," she said.

The slide was 500 to 800 metres wide and travelled a kilometre down the slope. Some of the skiers were buried as deep as three metres, said Ms. Dunn.

She said the group was carrying the proper avalanche-rescue equipment, including personal locator beacons and digging tools.

A park conditions bulletin said there was a considerable avalanche hazard in the area Saturday. That rating is about the middle of the hazard scale but includes the high probability that a person could trigger a slide, said Ms. Dunn.

"The risk always varies . . . when the risk is posted as considerable it's a note to skiers to be cautious," she said.

"There are a couple of instabilities in our snow pack this winter . . . that may have contributed and further investigation will determine [that]."

Saturday's slide was unusually deadly, said Eric Dafoe, public safety officer with Parks Canada in Revelstoke.

"We've never had an avalanche like this where we had a multiple number of people buried," said Mr. Dafoe, who worked on the rescue effort and flew over the slide as part the initial investigation.

"It was a huge slide," he said. "It covered the valley floor from side to side."

Mr. Dafoe said the slide began at a point between 2,200 to 2,300 vertical metres up Mount Cheops. It travelled for a kilometre and spread out half a kilometre at the sides.

The toe of the slide ended at about the 1,500-metre elevation mark with parts of it more than five metres deep.

Mr. Dafoe said the investigation will continue but was almost certain it was a natural slide.

"It [the slide trigger site] is so far from the bottom it seems unlikely it was human-triggered," he said.

Mr. Dafoe speculated a small slide may have occurred high up the mountain and as it came down, it dragged out snow from surrounding gullies.

Park wardens, RCMP officers and local ski tour operators and guides, backed up by four or five helicopters and search dogs, joined in the rescue effort.

Members of a Canadian Horse Artillery unit stationed in the park for avalanche-control work were also on hand to assist.

It was the second major slide in the area in a month.

Seven people in a party of 21 back-country skiers died in a much smaller avalanche Jan. 20 on Durrand Glacier, located about 30 kilometres east of Saturday's slide.

A memorial service for the victims was held just Friday in a Revelstoke church.

Ruedi Beglinger, owner of Selkirk Mountain Experience and organizer of the fatal expedition last month, was with his clients when it struck.

Contacted Saturday night, Mr. Beglinger did not want to comment on the level of experience necessary to ski the area where the students were caught.

"There is an inherent risk [in back-country skiing] which is very important to talk about, an inherent risk people have to be willing to share amongst themselves [whether] they have a guide or not," he said.

The area of Saturday's slide is not particularly remote, but Ms. Dunn said there were no groomed trails, "except people who have been there before you."

But no guides are needed to ski in the area and skiers do not have to register before setting off.

"It's a very, very popular ski destination site," said Ms. Dunn, media liaison officer at the national park.

Sgt. Brown said the skiers were not part of a commercial tour group.

Ms. Doyle said that at this time there are no plans yet to close back-country areas of the park.